How to validate 2 fields?

How to validate 2 fields?

Hi There

I am just about starting to use Zoho CRM. I have added 2 number fields where I have to let the user enter their percentages. How can I validate the fields such that the sum of both fields cannot exceed 100?

I thought about using formula field to add these two fields before hand. However the formula field is not available in validation rule?

Thanks

Manny