I tip based on effort. I gave the carhop at sonic $.050 today. I parked by the door. They took my order over the speaker, he/she walked it out to my car, I had the amount with the tip already in my hand and the interaction lasted less than a minute by far. I tip about 20% at restaurants. They are seating me, bringing drinks, taking orders, bringing food, refilling drinks, getting money, bringing change. I try to think about their target $/hr. If they are good they can cover a few tables and hustle to keep stuff covered. If I'm there an hour and tip 6ish$ on a $30 table for the wife and I, I assume she/he can cover 3ish other tables at teh same time. Will there always be tables, no, will there always be tippers, no, but lets say if everyone was like me, I don't bitch a bunch, I don't send food back, I'm polite, and I tip pretty much always. It is still their living, but if you're not cut out for it, you will get 5% (assuming its your fault, not the cooks/management etc) So that person with 3 tables getting $6/hr off each table (hopefully more) should be in the $20/hr range during the busy times, and hopefully more like $30/hr for 2.5 ish hours. The rest of the time there is a lot of behind the scenes work and slow times so it averages out. If a service worker can make $15/hr, that's probably right. I feel the amount I give is justified (duh, I've justified it to myself).
Back to sonic. He/she walked out, handed me food, I gave money, see ya. Now I don't have them for an hour and at a bare minimum 5 trips to my table with an open ended interaction(so 12-15 min of their hour on my top for $6), I have them for 45 seconds. So the sit down joint gets about $.050/minute they are with me, so does the carhop. My ticket at sonic was $5 with tax. I tipped 10%, but that person didn't refill my drink, they didnt' ask if I needed anything else etc, so my normal quarter to $.75 feels right depending on size of order and difficulty to deliver
I rambled, didn't proofread, sorry if my grammar and spelling are shitty. Not typing as much as I used to anymore