Calculate total valuation field

We are using the valuation a great deal to support the use of our bid alerts.  Currently, the valuation is calculated by the square footage entered.  However, for site verticals square footage doesn't make sense so we manually enter the valuation.  This is possible because the ValuationLighting, ValuationControls and Valuation fields are all editable on their own.

Here is the change request - make it so the Valuation field is not editable and will always be the sum of ValuationLighting and ValuationControls.  This will make it easier for our quoters to be consistent in the process and give us better results.