I wish i knew. You never know if those orders are entered the same way and the poor server is paying tax on your order, or what. When i was a waitress take out orders were a huuuuge pain in the tusch because they did get figured into our earnings and all the stuff for setting them up to go (plasticware, napkins, salt, pepper, syrup, jelly, catsup, mustard, mayo, butter, all that...) was in no particular order buried in a back room somewhere. Ironically enough, there was one company that used to order like 8 dinners at 2am and i just hated them; they always needed a whole spectrum of stuff and it would be an $80 - $100 order with no tip. I later ended up working for that company (and you better believe i never ordered from the place i had formerly worked
ETA: I usually aim for somewhere down the middle. Not quite the 20% i would normally, but something, at least. Having been a server, and living in New Jersey, i would never not give them anything at all. I just couldn't.