Weighting total on VA appraisal template level

I am struggling with one of our appraisal templates. I would like the object VA to be able to show the total of the KPA weightings. The user has requested that when he puts in the weightings he wants to see what the total is (almost like a calculator).

E.g. VA = 40

VB weighting = 20

VB wieghting = 20

E.g. VA = 60

VB weighting = 55

VB wieghting = 5

Just to show the user what number the weightings have added up to.

I have changed the template settings to allow the weighting to be maintianed (required) on VA level, but it still remains greyed out. Is there something i am doing incorrectly?

Thanks,

Chantal