Actually you're probably under thinking the process but if it's for LSR you're probably just fine the way you are. By cutting coils you've effectively stiffened the springs somewhat already. If you seriously want to get involved in the process there are two approaches. The first is find an oval racer or road racer with a spring scale or a shop that offers that sort of service and then once you know the rate of your present springs and the free height make a WAG at where you want to go. The other way is to borrow or buy a
good suspension book and follow the calculations. It involves the length of the suspension arms, the angle of the springs, and where the springs are located in relation to the pivot points of the suspension arms on which they're located.
As I said in the beginning you're probably fine just the way you are!
Pete