Dear Suenaga san
As I checked working time report of Mr. Yongyut in Oct, he worked 160 hours from total 176 hours.
In this case, you need to give the discount (deduct the monthly price).
Usually for every vendors, FSBT allows 6 hours in case actual working time is under total working time per month.
So the deduct working hours will be 176 – 160 – 6 = 10 hours.
Unit price per hour of Mr.Yongyut = 90,000/176 = 511.36 B.
So the deducted price should be = 511.36 x 10 = 5,113.6 B.