I'm sure Nick can provide a more scientific and precise answer, but what I did was just what you started. When the hull was glassed, both in and out, we went to our neighbor's pond. I brought a foam seat, some tape for marking, and Joy's trained eye. I tried different positions, sat still, paddled back and forth and found the right spot. I would recommend that if this is to be your daughter's kayak, you do the test with her in the boat. Every body/boat combination might yield a slightly different result. Example, I have skinny legs, so the center probably moves back a bit compared to someone with heavy legs.