Nitrous plumbing A1A...??..

https://static.summitracing.com/global/images/instructions/NOS-0022NOS.pdf
a LOT of good info as if you haven't read enough already:(

chapter 6.1 and 7, jetting/timing etc. there jetting chart show square jetting, thats a bit outdated, you can go smaller on F jet for a cleaner tune, 3>6 sizes but the cleaner you go the less the timing is. Engine combo efficiency is also related to timing. I guess at the end of the day perhaps a short shot enough to read your AFR as someone suggested is a way, reading your plugs is important and in some ways the only true way. Its sounds all a bit trial and error as I doubt if anyone can give a definitive answer to exactly how you should run YOUR combo a lot is dependent on it as I said. Its all in the article. Personally I would run square or even a tad larger on the F jet just to start till you get some readings on AFR/plugs and go from there, that way you won't have to worry too much about timing retards and how much is enough, 2 deg should do with a rich tune.