Ohh i can't remember but i know this but don't remember how . I will try to remember by using excel now. EDIT : sorry cant remember (and I don't try very hard cuz i am too lazy)
Eh, [your number]/100 times the percent you want. example: You need 45 percent out of 400 tc: 400/100 = 4 times 45 percent = 180 ;)