How do I measure for Casment Window?

I need some help measuring for a replacement double casement window.

Is it as easy as just measuring from the sill to the top of the window and then side to side? In other words, is the rough opening already exposed?

The current wooden window has the crank mechanisms screwed into the flat interior sill. The exterior hinges are attached to the slanted exterior sill.

The screens are held in place by clips that slide into metal inserts on strips of moulding that are attached to the vertical sides of the opening. This molding is set back about 1.5" from the face of the window opening.

With this setback, am I already looking at the box that the replacement casement will slide into, obviously after I remove the molding?

Thanks!

Replacing casement windows are a bit trickier than a double hung. You almost always end up doing a bit of extra trim work inside, ie stops ,quarter moulding and such. So going a little smaller isnt gonna hurt. Sounds like your on the right track tho....

do you have special forums for door installation illinois