First you need to figure out where your feet are going to be in the boat. As a rough estimate I'd figure about 48" in front of the rear of the cockpit opening. find out what forms that falls between. If it's close to a form your can use that form to measure. If it falls midway between 2 forms you might want to draw an imaginary form at that point. Whichever form you choose, put it on the floor and stand on it in your kayaking shoes, sandals, or wet suit boots. With your heels touching can you fit both your feet entirely on the form? If you can't it's probably time to think about deck modificatuions.

You want your ankles to be close to 90 degrees as you sit in the kayak. It makes it easier and less tiring to brace yourself. You don't want to have to reach forward with your toes to get them on the footbraces.