I have created a calculator to help me figure out how many rounds to knit for the foot after finishing the heel and before starting the toe for a CSM sock.
For the best results, you must knit a sample tube (you can later rip it out and reuse the yarn). This tube should be knit with the desired yarn at the tension you have determined is best for it, with the cylinder you are making the sock on, and have created a fabric you think will fit the foot circumference you want. It is best to use a washed sample in case of shrinkage (I use leftovers of the same brand of sock yarn when I have some to make the sample and wash it). At least let it relax off the machine for a while before counting rounds (rows) if you are going to rip it out to use for the sock. You can include negative ease if you desire and should do so if you wash your sample.
You need to know the length of the foot. You can look up what the foot length is by shoe size. I use this chart. Then count the number of rounds (rows) you get in 3 inches and divide that by 3 for the average rounds per inch (RPI).
Calculate how many rows are in your heel and toe. The toe rows are half of the cylinder stitches, minus the number of needles you leave in for work at the turn of the toe. I use a 64 stitch cylinder for my foot. I leave 12 needles in work (unraised) for the tip of the toe when I do my decreases. Therefore 64 / 2 – 12 = 20 rows in the toe. The heel is the same unless you make a deep heel. For example I leave at least 36 needles in work for a deep heel (I raise two less on each side the back half, so (32+4=36) for a deep heel on my socks. Therefore 36-12=24 rows for heel.
This calculator works for me, I make no guarantees, I just offer it as a tool. Remember, socks don’t have to be exact; knitting stretches a bit when it needs to and it will fit a range of sizes, so you just need a number to get you in the ballpark for the desired foot size.