As far as i know there is no adjustment but somebody may be along to correct me. The corners develop a natural curl from what i have seen on a lot of cabbies. I found on mine that the sort of leak you are talking about wasn't actually the seal it was the lack of waterproofer on the mohair. Mine was untreated when i bought the car and i had a bad leak by the corner seals, as soon as i treated the roof with proofer the leaks sorted themselves. Hope this helps.
1. There is adjustment for the roof (on the back part of each side of the frame where it hinges ..allen keys ..behind the rubber grommets)
2.. if it is leaking at the front corners have the rubbers dried ..try silicone spray or gel to soften
3 check the cables that go from back to front which have a spring at the back this keeps the sides of the hood taut
You can adjust the way the roof sits above the windscreen with shims where it is bolted to the car (Below the horseshoe), the originals are square shaped with the corners cut off (the ones on mine have holes and some are c-shaped about 1-2mm thick). However you could use anything similar like washers