Dear Pal,
Thanks for the question.
Why the range? – depends on the service level, for example :- you go to a fine dining or regular dining place -> end of your course while settling up the bill there is an option of gratuity 15% / 20% / 22% - Why every of the restaurants provide three options ? why not ONE ? cause of differ of the service – Why not we imply the same thing ??? pay 80 – if you had just regular service , 100 – if you felt like the service was better than normal , 120 – if they went the extra mile ..
My advice - > don’t overthink a lot -> there is a beautiful girl Infront of you -> treat her well and you get it back