EYZ, ignore all the flaming!
My advice in using the Airbus FOVE/LPC is to stick with the figures it gives you having made sure that all your selections on the F3 (T/o Perf) page are correct ie R/W state, anti-ice, contaminant depth, packs, optimum config ets. Airbus 'Grips' tell you a little about its logic, but it is clever and my take is that you mess about with it at your peril - unless, of course, you have good reason for doing so!
Cheers
mcdhu