Reading the plugs is of no value UNLESS you make a full run, drop the car into neutral, shut it off and coast to the end, THEN read the plugs. You can only get an accurate reading after a full open pass.

The bottle pressure is a function of temperature and " fullness ( to some degree) of the bottle. You will see it drop, if all things are equal, as the pounds of nitrous goes down. At that point you may need to add heat to it to bring the pressure back up. that is why you se some bottle warmers, thermostatcially controlled blankets etc. At the level you'll be spraying, it isnt quite as critical, but it should be maintained to the best of your ablility.

Doing your homework and understanding what to ask is a good thing..