Erm, didn't you get the Pythagorean theorem wrong in the OP? Shouldn't it be h=SQRT(x^2+y^2)? You squared the h.
Anyway, thanks for sharing Thales's theorem! Never heard of it. I tried this using an auxiliary layer as you suggested, and it worked well.
Fun note: I did this with another triangle of different dimensions just for practice, and the legs ended up matching the dimensions of my monitor! (mine is the first one listed here: [uh, this got blocked. oh well, don't want to bother with the permission.]). Loving my new QCAD, thanks for all your work guys.