When I fly the -400, I used the OPT APP for takeoff performance calculation.
There's some differences between the N1 calculated on OPT and PMDG almost all the time with the same assumed temperature.
I have reported to PMDG with this issue but with no positive answers until now. They just keep telling me that they're using the manual from Boeing, but my OPT program is used in real world operations. Hope PMDG can find the reason why there are differences between the real world data and what they build.