I'm in NYC, where sales tax is 8.25%. So I double the tip and round up. And throw in an extra $2, bcs I'm on a guest to brainwash all waiters into thinking, "Gluten-free folks may have a lot of questions and be really picky, but they're so NICE!! and they tip so WELL!!" (My way of "paying it forward" in return for al the groundwork laid by the folks who came before me on this path.)
When the bill is really small, I tip a huge percentage. I figure if you deliver my meal to my office, you get $5, no matter how little I spent--it's the same trip.
But likewise, if you're bringing two bags instead of one (bcs I've ordered for the group)--it's the same trip, so you may not get the full 20%; I'll tip somewhere around $12.
For the waiter in a restaurant, I tip the percentage or higher. If it's a diner for breakfast, and my bill is $5.12, I'll probably tip you $5. In a way, that's a minimum to me--you did the same amount of work on my behalf. Maybe more.
I have given substandard tips--once when I told the waiter that I'd order the sweet-potato fries just because I wanted to taste them. Then the order came, and it turns out my burger came with sweet potato fries (wasn't mentioned on the menu--I thought it came w/ reg. fries). My friend pointed out that the waiter should have alerted me; he should have know, even if it was only when he went to pick up my food--he should have offered me the chance to cancel the side order. She insisted--insisted! INSISTED!--that I not leave him a full tip. He has wasted my money! she said. So I tipped significantly smaller. Like 8% or something.
We hadn't left right away, and he came and asked me why I tipped so low. So I told him. He apologized and thanked me for the lesson: "I learned something," he said.