I want to replace a double casement over my sink with a 2 view slider.

I hear that casement openings can be tricky to measure, but mine looks
pretty straight forward.

The window is set back a little over an inch from the front of the
opening and looks to me like when I remove it, I'll have an opening
with four flat sides with some holes where the hardware is mounted.

From what I can see, it looks to me like I should simply measure from
side to side and top to bottom on the interior of the opening.

Am I missing anything? What should I be looking for?

It takes a week (from Tuesday to Tuesday) for my supplier to get a
window, so I guess I could take the existing window out and see what I
really have, then board it up for a week or so until the window comes
in. I know that would work, but I doubt the pros would ever do it that
way.

Any suggestions? Thanks!