You tune at half a tank due to the pressure in the tank and how it varies from a full tank to an empty tank. Tuning at 1/2 tank will get you somewhere in between. It'll run alittle rich when at full tank and a little lean when near empty. I don't start tuning until at least the 5 minute mark, no sense in trying to tune a cold engine. The longer you can wait the better but sometimes you know something is off after a minute or 2.