I see so many drivers throwing away track-time on rolling starts either through poor positioning or poor preparation.
Firstly, listen to the Clerk of the Course during the drivers briefing, if they have their own quirks, rules or things they look out for you need to know this. For example, some clerks are adamant that each column of cars is lined up over the grid markings as they approach the line so if you poke your nose out of the line the start is likely to be aborted. Other clerks can be less fussy about this aspect but might be more concerned with approach speed for example.
If you are on pole then you set the pace once the pace car has peeled off to the pits. You want to be at roughly the requested speed (usually between 40 and 60 kph) but pick a speed that suits you and your car. You don’t want the engine spinning near the top of its rev range as the moment the red light goes out you will run out of revs and have to shift. You want the engine speed to be just about to reach peak torque, as this will give you the most acceleration, but with enough headroom to accelerate cleanly without needing to shift too soon.
If you are further back in the pack make sure you are right on the bumper of the car in front, any gap you leave to the car in front of you is tracktime you are throwing away. As your speed will be dictated by the speed of the pack around you you may have more of a compromise in terms of gears and engine speed but the same rules apply: lowest possible gear with room to accelerate without shifting. Look at the lights and use your peripheral vision to keep a check on the car in front. Also be prepared to move out of line immediately to both defend your position from the car behind and attack the car in front.