Adding Allowance based on Basic Pay

The issue is that, I want to add Housing Allowance which is 30% of basic pay. When I go to New Allowance to add. I only find options asking for units and Qty and not on type calculations. Where can I find this option when adding an Allowance which is to be calculated on basic pay??

Sorry that this is not supported yet, but I will try to find time to work on this when possible.