I’ve gone into ethanol without oven purging since day one. or atleast the first time I winterized, which was many moons ago. open blast into a canning jar in warm water. adding ethanol to drive off the last of the butane. into the freezer.
With a CLS, I used to recover to 15" or so of vac on my receiver then use that vac to pull a measured amount of ethanol in.
I didn’t deliberately recover solvent to the cotton candy stage, but doing that more than once and “recovering” from that mistake by adding ethanol directly to the receiver was how I got to routinely using ethanol as my transfer solvent out of the platter.
If you leave too much butane in there, and release it all at once by spooning soup into rm temp ethanol, I can see it getting pretty messy (did I mention I’ve done this since day one? Trust me).
As for how to know how much ethanol to add, knowing the potency and weight of the input material helps. You sound like you have some historical data you could use to guestimate, even if you don’t have in house analysis capabilities.
If you weigh the ethanol you suck into the receiver, and the liquid you get out, you can have a number for your records 
Edit: and even use that number to tune the amount of solvent added.