How do I find the N2 RPMs?

Specifically, I want to change the capabilities of the engine I am using, but I can't seem to find the specifications on the engine. Its a specific one (P&W JT15D-5C), and I would like it to be as realistic as possible.
So what does the N2 rating consist of? Can I use some formula to figure it out from given information, or is it some obscure data that I would be hard pressed to find?
So what does the N2 rating consist of? Can I use some formula to figure it out from given information, or is it some obscure data that I would be hard pressed to find?