Horcrux socks

I've made significant progress on these socks, considering I bought the yarn on Tuesday and these pictures are from early Friday morning. But, as always with flap heels, I have a problem.

The heel flap is done in Eye of Partridge on 35 sts. So, here is the flap at 36 rows:

This clearly isn't a square, and the flap sure isn't long enough. Anyone know why I always have this problem? I always have to add on tons of extra rows to the flap, which leaves a lot more sts to be worked off in the gusset.

Just for fun, the sock so far:


And its mate. 50 rows in the heel flap to get it to about the right length.