How to set up benefit DBA with flat $ amount but variable %

I need to set up benefit DBAs for our employees who pay a variable portion of their benefits. We currently assign the $ amount at the employee level but this is cumbersome when the insurance premiums get adjusted annually. If this was set up at the DBA level it would only need to be changed here. I would like to set up the DBA with a flat $ amount for the monthly premium which applies to all. and show it as a percentage, but leave the % blank here and assign the actual % at the