Recently I've installed MPGuino in my Berta and it seems that there is actually the problem with instant vs current/tank FE.
I'was using 0.86v code and the problem was there, I've switched to "canadian" metric 0.81v code and everything is just fine.
I have to investigate 0.86v code further to find the bug, for now I'm using "canadian" version, but it needs calibration.
Soon I'll post pictures and description of my MPGuino installation process, it's slightly different, because Berta has a diesel engine