I'm working on another pair of socks (shhhhh) and I've noticed that many patterns I've looked at say something like "knit until 2 inches less than foot length".
Well that doesn't make it much of a surprise if I go up to the giftee and ask to measure their foot!
In my searching I found this chart that gives you shoe size to average foot length.
These charts from My Jewel Thief Knits give even more information on sock sizes!
I'm so glad I found those!