I always leave to find fuel: Trackside petrol is very expensive! I used to take jerry cans, but now I just leave the circuit and top up at lunch. I've normally done 3/4 a tank by then anyway, and it means I get to avoid the horrendous queues for lunch.
Just keep an eye on your rear tyres, as they'll go first. Usually LHS if the track is anti-clock, and RHS if it's clock. As long as you don't rock up with 3mm left, you'll be fine.