Pushing?
Obviously you’re not sucking hard enough if you have to push 
both @SanitatemDime & @Schwarzite are correct; there are three variables we can control here for any given distillation rig, so grabbing as much suck as you can and dealing with (adjusting) the temp & time to match will get the job done over a fairly wide range 
if you’ve got the math for that in an excel spreadsheet @spdking I’d love a copy.
I doubt he does 
. The guy blindfolded me at one point to go a mile down the street and make a left.
To close the topic and to educate everyone on the actual method of mating pumps up I will explain.
A 20cfm pump can perform better than a cheap garbo 40cfm pump. Pump quality matters. What you are looking at is the performance curve. Now another thing to remember is most pumps are supplied with a pump curve chart. Half of them are fake. The reason is becusee the Garbo pump manufacturers don’t actually know anything about testing. They throw shit parts together and don’t own a digital pump down curve analytical device. I do own one. It’s for verification and validity. The fake pump curves are obvious to anyone who understands how they work. Or claims where import pumps perform better than name brand pumps. Like when vendors claim they get lower microns than factory. It’s all a scam to sell pumps for 3x cost instead of a regular 15-25% margin. It’s all about pump profiteering. Not quality gear. (Great example a woosung Edwards 30 type Costs 800-1100 delivered to your door one at a time even white labeled, where as a Edwards wholesale is 4-5x the price - now imagine the cost when you buy 30 of these garbage pumps from over seas with near zero qc) Also the motors and rpms make a huge difference. For example there is a vendor selling re-cheapened woosung pumps they source from China that is built to a lower tolerance and cheaper build sheet - have been seen to have failing motors - where they test new at a reasonable rating but after a hour or three of use the motors burn out and or reduce Rpm and thus become inefficient. There’s alot to learn about pumps.
The method I use is the pump down rating at a giving micron rate. A quality pump that pulls say 20cfm open ballast can pull about 1.8cfm closed off at 50-100 microns(example) with a concurrent bleed allowed to establish the cusp point where the microns will not stop moving during the test.
I take the open surface area if a short path and make sure at micron rating desired I mate the pump curve at the lowest point of vacuum. The reason is because no CFM to liter rating will tell you what the pump can do and what it can handle from the boil off in the system. The only device on the market to be able to tell you this is a digital pump curve. (If you want to test your gear bring it by the shop and hook it up to my analytical pump devices for free). The only one on the market available for purchase is actually the Apollo vacuum controller that has it built in with that type of accuracy and guaranteed answer in performance. Prior to appolo nobody sold them and you’d have to pay 250k for some company to come install one in your factory, etc… Without that we only went off manufacturer ratings. This is how we manually found that many suppliers lied about the pump curve ratings.
So if I had 2.6 cuft of space in a large SPD(from fluid height to top of head, sometimes include condenser) , and my vacuum each was 1.8 CFM at desired rating, then I would pair two pumps up. Now two pumps at 1.8 CFM at desired micron rating does not equal 1.8x2. it’s more closer to 1.8x1.5-1.75 so in theory we make sure we exceed this requirement in the system with a abundance of evacuation during boil off.
This process of mating - exceeded - is actually what makes short paths run very very fast. And at lower temps. So basically you could have the best SPD out there and you’ll never hit highest speeds or lowest temps possible if you attached shitty pumps to your setup.
That makes sense, use the cfm rating at the desired vacuum level to appropriately size vac pumps.
Is the goal to be at least 1CFM (at the “bottom”, as measured by dig pump curve): 1 cuft?
so you take the open surface area, and measure it in cubic feet 
you’re right, it must take a special device to make that happen.
using the pump curve from the manufacturer (if you can trust it) makes good sense.
just like looking at a chillers cooling capacity at your desired temperature, understanding a pumps ability to move molecules at your desired vacuum depth is logically sound and the right way to do it.
Given that used pumps don’t come with curves, and your admonishment not to trust most curves anyway I don’t believe you’ve changed the way most of us are going to ball park our pump selection.
confusing area and volume doesn’t add any weight to your words.
I see you trolling me. Nice try.
Post them then oh vacuum god
Bestow upon us the knowledge of the ancient masters of suck.
He s not trolling he is yust stating that
We don t really know yet How to calculate best cfm for liter of flask
Since every crude has a diffrent composistion we won t be able to either
Yust for refrance what sizing if pumps are you offering on spd pakkages ?
Confuses volume and area.
Sure that it’s my fault
I’m not even sure what you’ve asked me.
And to rogue lab I’m not sure what your asking. I sell about 10 types of vacuum array setups probably and they each perform differently essentially. Some for basic uses. Some for diffusion work. Etc…
Spare a poor knave a single beam of concept from the shining crown that sits atop thy head oh honorable king of suck? Oh Mighty vacuous one?
What you say makes lots of sense. In my opinion to correctly size the pump one have to ask a question, what is the reason we run vacuum in the system. I am aware of 3 answers:
Getting lots of CFM pump is about the same why every Asian vendor call 3 way receiving adapter a cow shpae (because some newb cant type correctly). When beginner chemist purchases Glass made in China at 20% bargain and every joint is leaking air the only way to overcome an issue is to purchase high CFM pump to compensate.
If you run US or Canada made glass (not China shops like AI, USLAB or BVV) your chances for bleeds are very low and therefore you do not have to concentrate on #1, but #2 and #3. At this point one must calculate its vapor pressure and amount of vapor being produced at certain temperature from certain mixture from certain volume at certain ambient temperature less glass thermoefficiency at given thickness and composition in normal atmospheric pressure. Then take into the equation what spdking is referring to as chart. The pump CFM or LPH should be able to handle it or else. Basically at lower CFM you boiling point will increase during the process, but flow will still be heading to the pump direction unless pressure will so great that it will start popping out joints. It is not very simple, but truth is you do not always require E2M28 or 30 when you run on quality glass. If you look for quality EU glass made in Canada look no further [https://labcradle.com](http://labcradle.com in Toronto) all prices in Canadian dollars, guys from US don’t forget to change the currency you may be surprised.
I guess for starting chemist it is much easier just to spend extra $$$ for higher CFM pump than get into all this calculations and understanding the process.