7875 + 5.9 + E85 + Megasquirt

8AN Feed and return (1/2"). I've thought about the return being a bottleneck somewhere, but not sure where. I might check my connection at the return on the gas tank to see how it looks.
Yea -8 is plenty big as long as there’s no bottle neck somewhere. In your log with the vacuum reference connected I see 7.3 psi of boost and 62.1 psi of fuel pressure. If your base FP is 58 and your reg is 1:1 you should have 65.3 psi so the reg is not bumping it up or you don’t have enough fuel pump. Maybe the MS is seeing this and trying to compensate.