Validations for sum value

Hey,

I have a problem with validations:

I have in a product repository a multi-valued qualified lookup into an 'ingredients' table.

In the 'ingredients' table I have several ingredients and as qualifier the percentage of its share in the product.

So now I want to make a validation in the 'product' main table checking if the sum of all ingredients' percentages is 100%.

I haven't found a sum formula though, yet.

Can anyone help, please?

Thanks,

Chris