I would work on the same 100 baht per hour (or part thereof) ratio.
So your 2 hrs of good company = 200 baht tip + the drinks you buy her should be enough.
But it is really up to each individual.
But, then again you don't want to fark up the whole topic of economic expectations etc.
The Japanese and Korean tourists have done this in many bars in Soi Cowboy (like Baccara) where most of the BG's now expect the same 3 - 3 - 3 from every customer now for a ST...no offence meant, just a blunt hard fact of reality as an example.