Lats say you had icing in final segment, but not in second segment.
In this case, you would come up with 2 limiting masses, 1 for 2nd segm. and 1 for final segm. (don't know how your RPM lokks, but something like that).
Wouldn't the lower of these two mases simply be your MTOM vs. OBST? Methinks, but maybe your perf. documentation does not differentiate.
Brgds,
Empty