loop increment decrement

I am making a program that can calculate GPA but I cannot write soo much conditions to calculate GPA with decimal precision.So i was thinking if I could make a loop to add decimal values after checking my entered value it would be easier.For example:If i enter 76 percent marks the loop checks for 6 that is above 70 and than add 0.6 to 3 which is 3.6 and this way it calculates my gpa.....