The way I see people just panic and hit things without reading is so bizarre. Just breathe. Pause. Read the screen. The line can wait. You are not being a burden on those around you by taking a moment.
I read the screen and panic cause I have to do calculations in my head instead of having a reasonable tip option. Then, I end up tipping too much anyways cause I'm sitting there, staring, and they know I don't want to tip them as much as they want.
Move the decimal place one to the left of the subtotal. (Don't tip on taxes lol.) Say, $59 becomes $5.90. Then multiply by two. So tip roughly $12. Even if you mess up and round down when you shouldn't, at worst you'd only tip $1.80 less than you "should".