we all know that there's different ideas of tipping in different countries, often more to do with minimum wages rather than culturally (but not always), and it generally makes sense that if their employers are legally obligated to pay them a high enough wage to not be dependant on tips then tipping should either be lower or, depending on quality of service (or if against the law or restaurant policy), nothing
but whenever tips are given, for whatever reason, everyone always seems to calculate them by percentage of the bill. it doesn't take a waiter any more effort to write down and deliver a £10 meal than a £5 one (one might be very slightly heavier or more awkward to carry, but even then it's not necessarily the more expensive one), maybe it takes the cooking staff more effort but do they even get a share of the tips? the size of the bill has very little correlation with the amount of work the waiter needs to do, it's perfectly possible to have a high bill with a waiter who's had to do very little and a much lower bill with a waiter who's worked much harder
granted you can add some extra if you think it's too low for how hard they've worked, how friendly they've been, etc but it's always initially calculated from a percentage. just doesn't seem to make a lot of sense really