I really never thought about calculating it, but what I do on the master bus is this:
1) lower the threshold so that there is an insane amount of gain reduction at all times
2) look at the needle/GR meter
3) change the release time until the needle/GR meter moves in time with the music (however I define "in time" depends on the song and my ears)
4) then bring the threshold back up until I have my desired gain reduction (usually between 2 and 6db)
Attack time is usually 30ms (on the SSL G-Series compressor) to keep stuff punchy ...
this sounds good, ill try with a song i'm working on right now, i'll post if i get something interesting, thanks!