In cases where there is the ++ (i.e. 10% service charge) I don't tip. In other cases proportionally, usually not more than 20-50 baht. My taxi ride is usually around 60-65 baht, if it is a driver that starts the meter directly I always give 100. Taxi's are absurdly cheap, these guys make little money, work hard and the rates have hardly changed in the past 20 years or so.