Measuring Casement Opening for Slider

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

I measured for the 12 double hungs (6 different sizes) that I replaced last year and got them right, but I hear that casement openings can be tricky. It looks to me like I should simply measure from side to side and top to bottom on the interior of the opening.

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.

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 you guys would ever do it that way.

Any suggestions? Thanks!

For you, clean out the opening measure and then board up.
We Are pros , we take risk, we are responsible if the measure is wrong.