Tipping in Thailand
Among Thais, tipping is not practiced except perhaps leaving the loose change from a restaurant bill or rounding the taxi fair up to an even number.
There is no reason for you to change tipping practices or to feel odd about it.
In hotels, restaurants and bars frequented by tourists, tipping is more common and even expected by some. This is particularly true of "high class" venues.
If you elect to go along with this, there is no reason to be extravagant about it.
Depending on the service, 0-40 baht should be fine. There is no reason to give a tip at all in an establishment that has a service charge, as that is supposed to be the tip.
A minority of Thais who deal with foreigners not only expect a tip, but hint or even ask for one. This is not Thai style and, in fact, it is rude.
The best thing to do in this situation is to smile and ignore it.