well usally you can get away with 14 degrees initial timing on a N/A 302. Now when you run nitrous you are supposed to retard 2 degrees for every 50 hp shot of juice. Now that is a very safe number. Aslong as your car would be able to run 14 degrees N/A with no detonation then 10 degrees initial would be a safe number for the juice. That being said the only real way to know would be to read your plugs right after a run to tell if thee is any detonation on the juice.
here is a link to what plugs should and shouldn't look like from a nitrous car. http://www.dragstuff.com/techarticles/plug-pictures.html
Also for every 100 hp shot you should run 1 step colder plugs. I run a 150 shot and I run 2 step colder plugs.