Nitrous on a 340 100 shot

The reason you pull timing when on the spray is because the fuel charge is more dense and doesn’t need as much lead (time) to ignite. It’s not just as simple as pull X number of degrees for 100 shot. It takes testing and finding the value that makes the engine happy. Start low, pull 2 degrees per 50 shot, then read the plugs. Evaluate and retest. You’ll find a number where it makes the best power (mph). It will be less than your NA tune up.