How this is calculated
tip = bill × tip% · total = bill + tip · per person = total ÷ people
Per-person amounts round up to the cent, so the group never comes up short — at worst the server gets a few extra cents.
Tip on pre-tax or post-tax?
Convention (and most etiquette guides) says the pre-tax subtotal, but tipping on the post-tax total is common and slightly more generous. If you want pre-tax, just type the subtotal as the bill.