The following application is titled as a CG locator. If you look at it's output, you'll see that it also tells you where the NP is. It's quite a useful tool. I run all my airplanes measurements through it. Used to figure all the stuff out with pencil and paper but this thing is much faster and doesn't make mistakes.

The measurements needed are very easy to do and take just a couple of minutes. The value of those few minutes simple work is amazing.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

BTW, no matter what safety margin you choose and no matter what CG it suggests for that safety margin, the NP will be the same. As it should be since the NP is dependent on the planform.