Validation rules base on formula on other cell
 

I have a student grade calculation sheet.
I give user to change the ratio of exam and home work.
For example: Midterm : 30%, Final: 50%, Home Work:15%.

I would like to validate the total ratio should not more than one hundred
percent.

My problem is when I set the total ratio cell validation rule does not over
100% which only works when I manual enter a value more than one hundred
percent

For example user enter Midterm : 50%, Final: 50%, Home Work:15%.
The validation rule does not triggle


If user enter the value on home work and exam more that 100% the validation
rule does not triggle.

Are there any solution for this?

Thanks in advance,