AlpineGeoff

Answering this from a construction perspective: No it does not make sense to run rafter perpendicular to the existing. Running rafters this way, you will have to beef up the existing rafter to carry the load of the new perpendicular rafters, and also need to post under the new barge to carry the new rafters. You would also need a collar tie, and a structural engineer to figure this out. I would run rafters parallel and either have a different pitch on the side that bumps out 6', or run an eyebrow roof off the new main roof to cover the bump out.

Most standard/production type vinyl windows are actually slightly smaller than their stated size, so the rough opening is the window size. A 5/0 x 4/0 window (5'w x 4'h); would have a 5' x 4' RO. However, not all windows fit this. Some RO's are 1" over stated window size. As solver stated, check with your window manufacturer.