My kit is Tribute. I don't have a coupe but the principle, ie how the rear shell mounts to the frame, is likely the same as yours.
Pond's car isn't Tribute so potentially a different design.
Ir's quite obvious when the boot extension panel is in the right place. It fits snug against the back panel of the z3 then bolted in place. Trying to fit or line up the rear shell without first anchoring the boot extension is a hopeless exercise.
So it's mega important, on a tribute kit, to get that boot extension right first.
Then it's also quite obvious when the rear shell is in the right place. It rests nicely on the boot extensiion. On my kit the gaps between the wheel arches steel frame and the grp shell are all over the place but here is no gap between the rear shell and the boot extension. They're bonded and bolted together. Also the B pillar lines nice.
if you have those 3 points lined up without gaps, ie boot extension and B pillars lined up nice and fitting snug then the shell in the right place.
Once the rear is right I wouldn't try to fix the leading edge of the roof by moving the back again.