Mileage Calculation Form

I'm working on creating a form where truck drivers can calculate their mileage to and from job sites. What I have now is two lists, one is a "to" list, where it lists the Job Sites, and one is a "from" list, where it lists the destinations (usually to Rock Quarries). These two lists are being referenced in the user form as combo boxes. An additional list holds the distances for each to and from possibility.

What I would like to do is when the user selects a to and from location from the combo boxes in the form, the form references the mileage distance from the list of to-from possibilities.