When you multiply W simply, you defeat 3/4 of the
deltaW the models may be varying for corners cases.
You should be consistent in how you define W; I prefer
to always use the drawn layout actual dimension and
presume this is the foundry modeling folks' basis.
w for actual drawn width, m for actual number of drawn
fingers.
A delta-W variation modeling error could help or hurt
you, when it comes down to cases. "Two wrongs make
a right" style. According to SPICE. But at least proper
geometry modeling eliminates false mismatch terms from
the picture, leaving you with primary values to work on.