I dont think I'd worry too much about the amount of fuel you've got (performance wise). I usually need 2 passengers in my car before I notice a significant drop in performance. Last time I went to the track I used 1/4 tank to drive 4 miles!! Make sure you have enough :uh-huh:
Altho I've only ever run on 16" tyres, letting then down is certainly the way to go.
Launching is up to you and how much power you can feed to the tyres without them spinning. There is no "rule" as to what rpm you should launch at, just remember that high revs = good revs. Your mission is to find the point between 2500-6500 where the wheels dont spin, then feed it more.
Let us know how you go