I tip 20% or $1 per item, whichever is more, and then I round up.
So if it's $17 for 5 items It's $3.40 ($4) or $5, I'd tip $5. So I basically always tip $4 to $6. I only get delivery from Dominos, so I'm not sure what the more expensive places charge, but I'd tip to go along with that. I think Z Pizza is like $20 for a large pizza and they charge a delivery charge, so if it was $25 including the deliver charge. I'd still tip $5.
But when I'm drunk I usually never have $1 bills on me and tip in multiples of $5s. So if My bill is $21, I'd usually give $30 cause I don't think $4 is enough.. even though it's only 20 cents shy of 20%.
Ok, too much math, not enough work.