Math in custom properties

For linear segments I add a custom property called [Manual Subtraction] which is a number field which I use to make manual subtractions to wall areas. Typically I'll count off interior doors and multiply it by the typical door square footage to make deductions for wall sheathing and insulation. so in the [Manual Subtraction] property I would add somthing like 13*10 to deduct 130 square feet from my items. It doesn't look like custom properties allow for any calculations at this time.