25 scfm is the max you’d run on the 59025 (series version of the EXT420) Your chiller is a choke point too. A lot of guys bite the bullet and go with a 15 hp screw compressor. (expansion capability) keep your air quality at ISO 8573.1 Class 4 or better for longer air drive life. LUBE YOUR SHIT. Also, try not run over 60 CPM to pull too much vacuum as that really tears up the Haskels which are built to be serviced and have about a 3 --5 million cycle life before a major reseal is done. I’ve seen pump cooked in hours of use… Volume, recovery pressure/vacuum and recovery tank back pressure are all factors Lower drive pressure and higher inlet pressures are what they like… Bottom line 7.5 is more than enough as long as you are running under 90 PSI drive