Nobody wants to make an error when measuring for replacement windows. Let's face it - they're large, fragile, expensive, and sometimes extremely rare. Here are the measurements we take:
The outside edge of the frame.
The inside edge of the frame on the perimeter of the gap between the frame and window sash.
Edge to edge of the glass & metal sash/trim bordering it.
Primarily for fixed flat & vista view windows, this is the measurement of the portion of the window that sits inside the rough opening.