Oh, the answer to that is easy. If the person poured you a drink, brought food to your house, parked your car or took your order at a table and brought your foo

Oh, the answer to that is easy. If the person poured you a drink, brought food to your house, parked your car or took your order at a table and brought your food, base is 15%. If they were bad at it, lower, if they went above and beyond, more. Thems the rules, they've always been the rules, that's the social expectation. Anyone brow beating you into some other set of rules is trying to fleece you. They're saying "you're meek and you'll take it." The proper response to someone treating you like that is anger, not shame. You should feel absolutely no tinge of guilt.