For dirt starts you need to be as far froward as possible to weight the front end but for concrete starts you’ll need to around the center of the bike to help the rear end to get some traction on the slippery concrete.
Choose the highest gear you can and use the clutch to keep the revs high without bogging the bike down. You need to grip the throttle so your wrist
Tips to get a holeshot in motocross and supercross