FE Hoppy,
I have seen some famous errors made by performance engineers who did not take into account Vmcg limited V1 in calculating available assumed temperature thrust reductions.
A "same only different" problem used to arise with the RR powered B747SP on very short runways, you could only use Rating 1, not full rated thrust, because the full rated thrust Vmcg limited V1 could not be accomodated in the balanced field length, but the reduced Vmcg for Rating 1 was just enough that a solution could be calculated, and there was no way of using an unbalanced filed length, that only comes in to play with very long runways.
Tootle pip!!