[ILLusion]

The spring is non-linear. I can not answer this question for you, nor can most users here unless they have a degree in mechanical engineering or some form of metallurgical physics and have accurate distance measurements of the different coils as they change through the length of the spring so that they can calculate the varying tension through its stroke. You are asking a very difficult question with no easy answer.

If you want to do some hardcore research on your own, Guarder uses oil tempered Si-Cr steel for their springs. The tempering is different per spring. The main difficulty is the varying coil tightnesses through the length, and thus, it is non-linear. Your equation is not accurate for this case.

It would probably be easiest to just attempt trial and error: clip off one coil at a time and try the gun after each clipping. This is how most do-it-yourselfers do this. However, realize that by doing so, you must also look at WHERE you are clipping coils on the spring. If you cut the loosely spaced coils at the front of the spring, FPS reduction would be lowest, and will likely present high initial resistance for spring wind up.
Likewise, if you cut the tightly spaced coils at the rear of the spring, FPS reduction would be highest, but a reduced compression strength at the end of the stroke may "throw" your sector gear in to overdrive. You will be affecting the intended power curve by attempting such modifications without education.

This is not something I would attempt.