If you have pressurized CO2 (a canister), which means you have a regulator, and you said you have a needle valve, then you should just have it on a timer. A solenoid is usually a black rectangular piece attached to the regulator before the needle valve, and it has a cord. When it has power, it opens the circuit, and CO2 will flow. With no power, no CO2 will flow.
So, if you have your lights on a timer (8 hours on, or so), put your solenoid on the same timer (via a power strip), and you're set. No, you shouldn't have to adjust your needle valve at all, assuming you have the correct amount of CO2 entering the tank. And as long as your fish don't spend the later hours of the lighting/CO2 period at the surface, gasping for breath (yes, I'm serious), you're golden.