Nitrous Tuning

What's the engine/car package again?
I've always understood that the goal is maximum cylinder pressure at the right piston/rod/crank position with no loss of combustion control. More advance can cause the cylinder pressure to go too high too early which really stresses the engine components and also causes a lot of power robbing. Hence my question about the engine/car package. IMO the retarding of spark has to do with that rather than avoiding detonation. Detonation comes from liquid fuel (unsuspended) which is much more likely to burn in an uncontrolled fashion in a nitrous engine.
I would go 1 step cooler for each 100hp depending on the head material, chamber design, and ignition. I'd also suggest you might want to give Monte on Moparts a PM shout. He is THE sharpest nitrous guy I've ever known and can get you in what I'd consider the best place to start. I plan to buy the entire system for my race car based on his recommendation. It's early in the season so he may be able to answer you.