Should be more simple than setting the milage field in table 1. This is if you are not going to alter the milage in table 1 and it is a constant value for a city defined in table 2. This will prevent duplication of data which is what most developers strive for.
If the tables are related then adding the milage field (field 3) from table 2 to the layout for table 1 instead of the number field should work.
Read up on creating relationships.
Thanks for getting back to me, I followed that linked thread and it had almost exactly what I wanted to do... but i posted a comment on that thread with the additional question that I have regarding the ability to control the fill with a button..
And what you want is possible. See that thread for my response on how you might do so.