I agree. As soon as the pressure starts to drop below 750-800psi, it means you're out of CO2. CO2 is stored as a liquid in the tank, and as long as there's liquid left, the pressure will remain at the 750-800psi range (depending on the temperature).
The 200psi remaining is only a tiny amount of CO2, maybe a couple ounces at most. It's probably not enough pressure for your regulator to function properly.