Hi Andrew,
I'm trying to keep the math and logic easy to understand. Let's say that our base boat has a 30ft mast and that the center of effort,CE, in the sailplan is at 40% of mast height plus 3ft above the hull center of bouyancy,CB, to the bottom of the sail. The CE of this sailplan is located at 40%x30ft + 3ft = 15ft above the CB of the hulls. Now our 10% taller mast on boat X will be 33ft tall with a CE at 40% of mast height plus 3ft from the hull CB to the foot of the sails. This measuerment is at 40%x33ft + 3ft = 16.3ft.
The ratio of 16.3/15 = 1.087 or about 9% higher center of effort in the sailplan of boat X.
If I ignore the height from the CB to the bottom of the sailplan and simply take the ratio of the heights to the CE in the sailplans based on mast length alone, I would take the ratio of 40%x33/40%x30 = 1.1 or a 10% higher CE in the sailplan with the 10% taller mast.
You are right in that the CE only moves upward half as much as the top of the mast did but since we are taking the ratio of the two CEs, the 40% factor cancels out. So the CE moves up the same percentage as the mast top moved up.
Good Sailing,
Bill