I'm taking off the storms and painting the exposed wood, not wrapping it.
The windows were measured just inside the sill and made 1/2" shorter than the rough opening. That measurement seems to be correct, but had I thought about not using the head expander before hand, I would have added a 1/2" to my RO measurements.
I'll admit it, probably a rookie mistake, but I just followed the various directions I found for measuring windows. For example, at
"If your window sill outside slopes downward for water drainage, you have to make sure that you measure from the HIGHEST point of the sill."
Anyway, I (temporarily) installed 1 (of 9) windows today and this is what I did:
I screwed a 1/2"x 1/2" piece of stock to highest point of the sill so that the top of the window is behind the upper stop - no head expander needed.
I then ripped a piece of 1/4" x 1 1/2" vinyl lattice in half and tacked it to a piece of 3/4" stock which was ripped at 15 degree angle and the correct height to support the front of the window. I then screwed that to the sill just inside the side stops. Now when I snap the vinyl sill expander into the track on the window, it will be tight against the vinyl strip that now hides the gap, while the wood prevents to bottom sill from flexing. My next step is to insulate under the window, install backer rod along the top and sides and then caulk, caulk, caulk.
Yes, there's some extra work required, but it'll be worth it not to see the head expander.