You want real device measurements, calculations are only
as good as the data, assumption, simplifications behind
them.
I've been inside foundry and I've been fabless and the only
thing that makes sense to me is "trust noone". Especially
when you're stepping off the beaten path. If you can find a
"model vs WAT" report but in it, can't find anything showing
negative Vgs, then don't trust that the models match the
silicon. If you can't get the report, then don't trust the
organization.