Number one thing to remember is that timing kills. Pull extra timing, it won't slow the car that much. Once you get used to it and have had some time reading plugs you can start adding 1 degree at a time. You get the very best reading when the plugs are installed right before the pass and pulled right after. Kill it on the return road and pull a couple plugs. Driving around before and after the pass at part throttle will totally change the plug reading. Are you running pump gas? What is your compression ratio? What is your fuel system consist of? Do you have an MSD box that can pull timing? What manufacturer made your kit?
You need to purge the system right before you make a pass other wise you will have a rich bog as you leave the line. You can use a purge or setup a button to activate the system and purge it into the engine. If you purge into the engine you have to bring it up to 3,000 before purging or you could cause damage. I always just ran a purge solenoid to vent at the base of the wind shield. Then I could see if it was just air or nitrous. Go 1 step colder on the plugs. I wouldn't mess with a fuel pressure safety switch. When I first started running nitrous I ran a FPSS and no purge. A bottle heater is helpful but not necessary. Depending on the kit, you need to keep the pressure above 900PSI and typically below 1,000. If the pressure is low it will run fat. If the pressure is high it will run lean. Don't use a torch to heat the bottle. I would put my bottles on the passenger side floor of my truck and run the heater aimed at the floor to bring the pressure up. Or run the A/C to bring the pressure down. The heater will come in handy if you are running at night and the weather is cooling off. With the flip of the switch you can bring the pressure back up while in the staging lanes.
Below is a picture of my car 10 years ago, or more, running 10 teens at 128 with a 357SBC on a 175 shot. It ran 11.20's on the motor. It ran 9.71 at 143 with a 175 shot on my 427SBC. The 9.71 tune was real lean. The fuel jet was 16 sizes smaller than the nitrous jet and it only had 22 degrees of timing. The 175 shot on the 357 was according to the manufacturer, NOS, and the fuel jet was 10 sizes bigger than the nitrous jet. To get it to run clean I ran 30 degrees of timing on that tune. The tunes were much fatter years ago. I ended up reducing the fuel jet by 26 sizes from years ago to recently on the same 175 shot in an NOS plate. Also pulled a bunch more timing.
You sure are right about NOS systems being fuel rich. The first time I got on the dyno with a cheater kit we went down 10 jet sizes on the fuel side and the A/F was still at 11.8, when I went to the track the next time with the new tune the car dropped from 10.39 down to a [email protected]
(small load). Kinda sucked that day as that was the end, the slicks were not having anything to do with more passes like that. Felt like I was racing on Lay's potato chips the rest of the day.