There are lots of variables. Is your car FWD, RWD, 4WD? Where's the engine? Is the track dry or wet, or are you on dirt or snow? What kind of tires are you running? What kind of assists are you running? If you are running any traction control, you can basically floor it, the computers will do the work. If you are running no traction control, then you have to weigh all those variables.
The individual car will make a difference too. When running no traction control, I'd say start by flooring the pedal, and if the wheels spin, try again at 1000rpm less. Until you seem to get a good amount of grip off the start, fine tune it even more if you can.