It's all done on height... x mm from the top of the stanchions when the forks are compressed and the springs are removed. If you look on the Windwave website there is a chart for all of the forks.
I've always found it best to compress and extend the fork a few times when filling it up. Without doing this the fork has felt too fast so I've had to run lots of rebound. It seems to ensure the damper cartridge gets full too.
Yes it pumps the air out trapped in places.
But don't do it too fast as it can blow the oil out.
Vernier callipers come in handy for oil heights, or a skewer with half cm marks on them.
I know on the turn of the 00's and earlier you can adjust the oil height to tune the spring rate slightly to compliment the metal springs as the air is still acting a bit like a spring (it's in their Tech docs)
Some forks do use XXcc of oil. Certainly Marzocchi turn of the 00's air/oil forks.
Anyway just a thought, no idea what era your forks are but if not too new then try this,it'll give you some insight anyway .
http://www.retrobike.co.uk/gallery2/d/1 ... f72e652733
I think there are some other techy ones for other eras on the net as well.