Thanks, I found that equation already but since pressure is not constant and I don't have illustration how the pressure is varying I couldn't get a function for that required integration. My only option for now is to get the trim from elevator and used that data to get the CP given by existing CG. The reason I'm doing this because most F-18C including mine I believe are nose heavy based on manufacturer's data, the plane flies a lot of speed at 0 angle, if I want to slow like put the throttle to half then I have to set the trim. However I can say that the plane is very solid and stable even on windy. I believe I can reduce the speed if I get the perfect CG which is 25%MAC from leading edge of MAC.

Johnnie Red - Yeah, my worry is when I move my CG backward based on calculated value of CG if I don't have the right location of CP, I might affect the stability of my plane. That's why I also want to know the limit of acceptable CG to make sure if I really need to make an adjustment. Thanks again!