Hi Andy
I fitted the winglets first and just held them in position by hand using my eye to get the right position. I used an erasable marker to mark the first hole and then drilled and fixed this loosely. I then swivelled the winglet into the right position, marked the hole, moving the winglet out of the way before drilling the hole. The plastic drills really easily so don't press hard or you will mark the finish with the chuck jaws.
The screen is very straightforward and I fitted mine outside BMW Oxford before riding home with the old screen attached to the tank bag. I found the simplest way was to loosely fit the support bar to the new screen, fit the lower screen bolts to the front swivel mount, then put the three plastic fittings in place on each side where the bar and screen attaches to the bike. You do need to make sure that you get the parts on the right side and the bar has to be flexed a bit to get it over the mount. It goes quite easily if you do a side at a time. I have just posted a full list of parts on a newer listing under "1200 GS" then "Adventure Screen onto GS"
hope this all helps.
Stability at 90/95 was great with no problems or issues at all. The screen is really rigid and does it's job far better than the standard screen, the tall screen from Nippy Normans and the NRA screen with the adjustable aerofoil, all of which I have tried. I won't be changing now.