Hi:

So your lookup table will have three columns, From, To and Distance? Or will
there be another table that you use for a calculation based on the selections
in your other drop downs?

If it is the first scenario, you can connect an expression box (for example)
to the Distance field from your secondary data connection and filter it based
on the selections in the To and From drop downs. If it is the second
scenario, more information about your intended set up would be helpful.