HEI has standards for air inleakage based on steam flow to the condenser, number of openings, etc. Try to get a copy of their material.
Rheovac makes a nifty unit that will trend lots of information about everything that is in the vapor stream being pulled by the vacuum equipment. Google them. They are in or near Columbus Ohio. I am out of the country so I can't put my hand on information about them right now.
rmw