Certain percentage of expense categrory as a business expense

Certain percentage of expense categrory as a business expense.

Say I have a expense categrory named "Vehicle Maintenance Costs". I have $1,000 worth of expenses in this category, but as I drive this vehicle 50/50 between business and personal, I can only write off 50 percent of the total expenses in this category therefore my business expense for this category should only be $500.

The ability to add a percentage of total costs in this category would be a nice add.