On the Sig Akromaster with the .20, I would go with 52' eye to eye and .012" lines.
No offense, but, Yikes! If you like 3 second laps, maybe. The 20FP on the Acromaster (about the size of many 1/2A stunt planes) will be an absolute *bomb*, CRAZY FAST. Not to mention grotesquely nose-heavy without a ton of ballast. You can fly a Nobler with decent results with a 20FP - a full-sized Nobler.
Super-short lines are not the way to go. I run .015x60-62's on my Skyray 35/20FP. You may get away with the .012s if they are kept in good shape but I would not feel comfortable with that. I would suggest .015x60 to begin with, and makre sure to put in enough line sweep and tipweight.
In any case, I don't think the Acromaster and the 20FP will end up being a happy combination. Might want to reconsider that, and use an older, baffle-piston 15 (as it was designed for), a 15 LA, a 10FP, or something like that.
Brett