I assume the Piper and all other aircraft manufacturers allow some slack in their numbers and quote the worst case
Figures from manufacturers are used to sell aircraft. They are usually unfactored and produced by test pilots. By that I mean a genius has gone out and flown the pants of a brand new aeroplane to land/take off in just that distance. That is why we add safety factors (there used to be a good pink AIC about this). In a knackered old crate with an inexperianced pilot you will struggle to even get to the factored figures.
Still if you do decide to ignore that, dont try to climb over the ridge at Mull, I did in 1996 when I had 100 hours total. In the club C150 when I took a mate there for lunch. I absolutely sh*t myself and we cleared the saddle between the hills by about 100 feet if that. Should have made the turn back out over the bay and gone round the hill.
Nice luch though.
Other advice, life jackets, ELT. Raft. GPS good weather an watch out for the wind. Even a clear day can be fierce windy and that generates huge rotor and downdraught.
Lot of good advice on this thread, dont write us all off as a bunch of old farts trying to prevent you having fun we all have your best interest at heart.