Looking at "How to Earn Miles" from the BMI website.
Thai airlines adds up as follows.

3.0 x F,A,P
2.0 x C,D,Z,J
1.5x -
1.0x U,Y,Q,M,B,S, H,T,K,G
0.5x V, W

Looking a possible booking with TP, the Fare code is - QJ3MGB1

I'm guessing I just take the first letter to calculate the millage multiplier?

Thanks.