I stopped growing around 15, I'm about 5'3" and about 120 lbs. My mare is 14.2 if you're really generous and she needs a trim she's also half arab, granted I ride western but she EASILY packs me around.
The thing that I put above the percentages and calculations is: do you feel awkward on her? I absolutely can't stand anything over 15.2 that was immediate reason for me not to purchase a horse when I was looking, however I have a friend who is actually about my size who prefers horses 16++ hands.
Bottom line is a) at 5'1" I doubt you'd look ridiculous on him and b) if its a lease it probably won't be permeant and you can get out of it.
Yep SorrelHorse my mare is 14.2 hands of spitfire when she wants to be
"I don't think he ever gave a thought to other people's opinions, which was just as well because they were often unkind."
-- James Herriot, All Creatures Great and Small