Hi Chris,
Not a stupid question by any means. First of all: for the 32m routes a 60m rope (not a 50m) will usually suffice, factoring in its elasticity, but sometimes just. So always tie a knot in the end of your rope. Secondly, there’s nothing wrong with your math where routes that go straight up are concerned. In the guidebook, when it says for example “45m route but 80m rope will do” this is usually on an overhanging, diagonal or zigzaging route, where the length when you go up is 45m but it’s less than that when you are lowered straight down. Does this make sense?
In sum: Don’t bring a 50m, bring at least a 60m rope; the routes with specific remarks about rope length have been tried and tested; and finally, always tie a knot in the loose end of your rope, as running out of rope is THE most common accident!
Best,
Aris