There's a thread somewhere that details the FT discounts, sorry I'm not going looking for it (IIRC it was in the News forum).
Google Translate on your phone CAN read Thai and gives quite good results on energy bills.
Our bills that were printed on 18/09, 19/10 and 18/11 show the FT discount as a -ve amount under the FT charge.
The discount is almost 1 Baht per unit if usage is below 300kWh, a sliding scale between 300 and 500kWh usage and no discount over 500kWh usage.
Our last bill (printed on 18/11) for 114kWh usage shows an FT charge of 106.51 Baht, immediately followed by the rebate of 104.93 Baht.
The bill printed on 19/10 for 303kWh shows an FT charge of 283.09 Baht, followed by the rebate of 156.05 Baht (just missed the full rebate by 3kWh ???? ).