Ok. $.06, and I'll probably publicize my ignorance, but here goes...

I'm 5"10; boat is 24" beam, about 12.5" tall ... I just measured the paddle and it is 86". For me, I would think another 2" on EACH end, at least. For you, maybe a little more if your boat is 25" wide. I'm basing this on what I read about using this type of paddle - fast cadence, shallow entry/exit angle, lots of torso action vs. "all arms", and hands down low, rather than being held high. I was just thinking that if you are beginning to build, it would be easier to build longer than the formula, then cut down the blades if necessary. Hopefully someone with more (any) experience will reply in case I'm all wet behind the ears. Caveat Emptor!