What most naturally follows?

How to install replacement windows
Take the width measurement from inside-to-inside of the window frame.
Take the height measurement from the bottom of the window frame to the top of the window frame.
Some people take the height measurement from the top of the sill.
This leaves room to attach a " header expander " to the top of the new window. Others, though, dislike using header expanders because they decrease effective window size, and complicate the trim process.