I do not know the nationality of the OP, but this is usually a question asked by US citizens. Until recently, being European, I have always been confused why Americans always are so worried about how much to tip. I was shocked to find out that in the US, the tip is actually a significant part of the staff income for restaurants in the US, which is completely different from how we do it in Europe. In Europe, you get a proper salary, protected by unions and the law. No need for a tip, although it is always welcome.
Here in Thailand I don't sit around trying to calculate percentages, just give some of what is left as change, like 20 - 40 baht, or for bigger occasions a 50 - 100 baht. For a roadside stall, no tip is expected. For maids and handymen no tip is expected, but I usually give the handymen something they can repair and use or sell, like an old aircon unit or a broken spinbike/treadmill.