Now I know this is a stupid question but I'm trying to get my head
around measuring up for windows for a timber frame opening.

When you're looking at the opening from the exterior does the top of
the cill (reconstituted stone) sit level with the bottom of the timber
frame window opening or is it the bottom of the cill that sits level
with it.

I need to work out if the window opening will be the final size to give
the glaziers or if I have to take the height of it and deduct the
height of the stone cill.

I've had two different thoughts on it so far so any help in what is
normal practice would be much appreciated.

Thank you.