Hi Mark
As I recall anything over 20" on a vacuum guage was OK. I would always wait at least 15 seconds beyond that before exposing in a small frame and up to a couple of minutes with my big frame (high volume pump and 66" x 96" glass size). The material behind your negative and print can also make a difference. I bought an Agfa colour print system which included a vacuum frame which used a thin polyester sheet instead of glass and the pump looked like an aquarium air pump but used in reverse. Not very high vacuum but it worked. With the plastic curtain, a rubber roller helped a lot. Withy any of them vacuum was broken when the pump was turned off so I think a foot pump and shut off would not be satisfactory.