On the E195 manuals there are tables to corect the N1 to N1 flex, but I don't know if our crews use to calculate it or not.
We certainly teach crews how to do it but what the airlines do after their pilots leave here varies considerably. In reality all the crew is doing by manually calculating the Reduced N1 is confirming they put the correct TO and FLEX temps in the box. The box won't get it wrong so if there is a difference it's down to miscalculation by the crew or finger trouble.