I calculated the Cp using a spreadsheet based on the Barrowman equations, accounting for the fact that the motor sticks out the back end by 0.46"
With this method, Cp was 23.15" from the tip of the nose cone
Needed a 14.25g nose weight on my rocket to get acceptable stability margin. I used a lead fishing sinker hung from the loop at the bottom of the nose cone for the weight.
Note that the Cp and weight quoted above are for the Estes # 3226 Hi Flier XL. Your weight may end up being a little different than mine, as I also made a few modifications to the kit (heavier shock cord, steel leader motor mount). But you'll almost certainly need a good lump of mass near the nose to ensure stability, esp with the E motors