If I understand correctly, the question is, how big of a cog can you put on the FW side of a flip-flop, if you're running an 85 inch gear on the fixed side?
If so, that will depend on the combination you use to achieve the 85 g.i., and the length of the horizontal dropouts. Figure that the addition of one cog tooth moves the wheel a quarter of an inch in the dropout, and you might have a usable inch of dropout in which to move it. So if you are achieving the fixed-side 85 g.i. with, say, a combo of 48 x 15 (86.4 g.i.), then you might be able to get an 18T cog to work with your chain and dropout length, giving you a 72" gear on the FW side. But it's far more likely that you'll only be able to get a 1- or 2-tooth difference, which will yield an 81" or a 76" gear, respectively.
If you were asking my opinion, I'd gear the fixed side lower for climbing and install a brake for the descents, eliminating the FW altogether. My experience with flipflops is that you quickly tire of flipping the wheel, and just find a compromise gear on one side or the other, depending on your preference for fixed or free.
Best of luck with your project!