fitst, can you tell one more time, why you are using proc_mode = PROC_LATE; ?
in this way all grapfical entities are moving a little bit "after" then physical bodies, where is the "plus" of this method?
second, using "slices" would be good for "slow-mo", i think
i tried another variables(not 0.01, but 1 or 1000), but i dont see any difference between this, so i'll think about your formula
int advance = time_frame/16;
int slices = integer((advance / MAX_NEWTON_ADVANCE) + 1);
int slices_left;
for (slices_left = slices; slices_left > 0; slices_left--){
NewtonUpdate(nworld, advance/slices);
}