Start out with a 100 shot, get it tuned in, check your plugs and press that button, Once you get a feel for the tune and adrenelen rush put in a bigger shot.
I think you will find a 150 to 175 shot will be more than suffecient to have a little fun and still be safe level of no2 to prevent destroying your engine.
You may want to stay in the 100 to 125 range if your converter isn't set up for nitrous. Too big a shot will baloon the converter.