My usual suggestion is go until it blows up, then back it off 50hp...
150 shot
.058N, .052F, 6psi, 900 bottle pull at least 8 degrees timing.
That's where I would start. Here's a couple decent places to get a baseline tune up.
http://www.robietherobot.com/NitrousJetCalculator.htm
http://www.dynotunenitrous.com/store/Scripts/Jet_calculator/nitrousjetcalc.asp
If you start with a N jet for a certain HP and step up the N jet, that increases the potential hp.
Approximate sizing of N jet for an HP.
75 = .041
100 = .047
125 = .053
150 = .058
I go the other way, take fuel away until the car car slows down, then back up one jet size. Then start dialing timing back in until it slows, back out one degree. Watch the plugs for speckling/detonation.
If you aren't familiar with juice, I would start with a 75ish shot and tune it. Step up once you get the procedure down. It's harder to hurt your stuff with a 75 shot. If you run it real rich, that's as bad as lean.
Run decent fuel, race gas at that level isn't required, just get good premium level fuel, 93 or 94 is better than our pump swill.
If you have an A/F meter even better! That will tell you if it's fat or lean.