How to measure when i can see studs

I am replacing some vinyl windows with new vinyl windows. I have all the molding etc. off. I can see all the studs. If I measure at 3 points for the width and height, how much do I need to take off the measurement? Thanks

1/4 of an inch off the width and height is considered standard cutback.

If it's a down to the stud installation using windows with fins then I like to allow 1/2" in both width and height. We shim the sill up 1/4" and that leaves 1/4" all the way around the window to be able to foam insulate real well.

For a drywall return (where there is no interior trim or casing) do you measure the width from drywall face to drywall face, add 1" (for 1/2" drywall), and then subtract 1/2" or 1/4"? For the height where there is drywall return against the header, but a wood sill, do you do the same?