One suggestion is to use something like gordian knot to determine a file sized based on the bitrate you want.

Ok, I'll try again. Roughly what percentage does 2 pass get you over 1 pass highest quality?

in 2-pass, you choose the desired file size. although it may not be exact, in my experiences [when doing anime episodes], they generally tend to be 5-10MB LESS than what i input. you input the file size during the 2nd time around [2nd pass].

yeah i filter in huffyuv, then compress to an mpeg 4 codec.