there is no optimum launch rpm for all cars, it will vary depending on the car, the drivetrain, the amount of power, etc. the easy way is to just turn TCS to 10 and hold the throttle down. i drive with ASM and TCS set to 0 all the time, so this is what i'd reccomend
sit down and gold all the license tests which start from a standstill, make sure you watch the demonstration, this will give you an idea of the general ways to launch a car and how different they can be. you'll also see (as in the very first license test), sometimes it's better to bounce off the limiter in first for a little bit than to shift as soon as you hit redline
i try and find the rpm where i can take off, and don't get wheelspin straight away, sometimes (stock awd or heavy rwd) this may be on the limiter, easy. on lighter cars and FF cars, you'll have some experimenting to do. now, as the car moves forward without wheelspin, hold the throttle steady and let the revs drop, to what point will be different with every car, but trying 1000-2000rpm less than the launch rpm is usually a good bet. now nail the throttle
unless you have way too much power or really crap tyres or the car just sucks (again first license mazda2) you should grab traction and take off. if you still wheelspin, slow down and study what the car's max speed is in 1st gear (with traction), and shift say 5-10km/h before you reach that speed. in cars with a crap clutch and 1st gear wheelspin, you'll want as much speed as possible before you shift, as you'll lose acceleration force whilst the driver changes gear, so you want to minimise slowdown
in some cars, you are better to launch in 2nd gear off the limiter. this makes me thinking back to license tests involving the spoon civic and dodge viper, where i tried my ass off to get within 0.005 or so of a gold, then tried launching in second, and i would have had to tried to MISS the gold
if you're super curious, sit down with a pen and paper and go the the test track and try launching your car in 1000rpm increments and see your results over 400m, write down what you get for each car that's important to you, so you remember when you race
i actually keep an exercise book on the coffee table for writing game crap down (level passwords, gt suspension settings, etc) but that's another story