thanks , Dje, but i still have a question about that, you mean i should do this 1000-1000*10%=900, 900-900*10%=810, 810-810*10%=729 , 729-729*10%=656.1, round 656.1 should be 656 , then 656-656*10%=590.4 , round is 590 , ... right ?
what about 20%, same above ,right ?
thank you !