I make a lot of socks top down with a gusset heel. Never use a pattern any more, just measure against my body. From fingertip to 2nd knuckle (bend your forefinger) is the length of the ribbing, tip of longest finger to heel of hand is leg length (sometimes I go a bit longer), I've finally memorized how to turn the heel (no tips here, I just memorized it -- put a marker in the center, knit two stitches past the marker, SSK, K1, turn, slip the first stitch and then purl 2 stitches past the marker, P2tog, P1, turn, etc.) Foot length is determined by trying the sock on and starting the toe when I get to my bunion.
Here's my tip: when making the heel flap, do the slipped stitches on the purl side. I can knit a lot faster than I can purl, so I speed up the process by doing P1, sl1 and then knitting back. I never could figure out why standard instructions make you K1 sl1 and then purl back -- it's so much slower.