Never adjust when wet.
Stick a poly bag under the saddle and use that in the rain
Use mudguards at the very least an arse saver. The key is keeping the underside /unfinished side dry as possible.
Never adjust when wet. It’ll dry out and shrink back and tear at the rivets
Adjust only a small amount if really needed once it’s dried out.
Apply proofide to the underside once a fortnight for the first 2 months then move onto once every 6 months do the top once a year.
My b17 fits like a glove the brown one broke in much easier than the black one. The black ones leather was much harder for much longer.
My swift ti has never fitted like a glove but still comfy but what it does do is not get any less comfy even after 7x 12 hr days off road.