SPF2010: Using a lookup for a default value

I have two lists: one is a list of customers, and one is a list of service calls.

The customer list has [Driving Distance], which is the distance from our central office to their office; likewise, the service call list has [Travel Start Time] and [Mileage] for our employees to document their travel.

Now, here's my idea: when an employee creates a new service call, if they had to travel, take a guess at the mileage. I've already tried setting [Mileage]'s default value to this calculated formula:

=IF(ISBLANK([Travel Start Time]),
    "",
    [Client:Driving Distance]
)

However, SharePoint just threw an error back at me, saying that [Client:Driving Distance] didn't exist. (I know it exists, but I guess I can't use it in a calculated column.)

Is there any way to implement my idea? If SPF2010 has a JSON API, I'm not familiar with it.