Get a bottle heater that wraps around the bottle when the bottle is in the car. You probably have a generator so plug the heater in, to bring the pressure up to where you want it. We set the pressure at 950 psi. Between rounds we use the in car heater that runs on 120V, also has a 12V wiring if needed but we use the generator. If you use the bath heater for your spare bottles that is good, obviously it has a thermostat to maintain temp. You will have to figure out what that temp is as not to over-pressurize the bottles. If you are in the lanes a long time the 12V heater wiring will come in handy. Wrap a thick towel around the bottle in the car to keep the temp and pressure from falling too quick.
DO NOT USE AN OPEN FLAME OR PROPANE TORCH TO HEAT THE BOTTLE. Have seen this too many times and I pray that nothing happens to the guy doing it. But I am sure that you already know this.
Have generator, then you have a battery charger to keep up the battery up between rounds, especially if you are using the 12V heater.
Make sure that your nitrous fuel flow gauge has at least a 3 to 5 foot hose to go into a fuel jug or make sure it will reach your fuel cell if it is mounted under the hood for when you are adjusting the fuel pressure to fine tune.
DON'T FORGET to open the tank valve as well as the main line valve...
You have been racing a long time so you know that you need a routine and stick to it, for the most part.
Have fun!