Calculating Relating Values

[mcinfly]

I have a drop down menu that is displaying results from a related record & table. What I'm trying to figure out is, based on the item chosen in the drop down menu, how do I calculate other related data into the current table.

Ex: I've got 2 files: Address & Bank

From my Address file, my pull down menu displays a list of that client's Bank accounts from the Bank table.

When the Bank account is chosen, I want it to calculate that bank account's routing number into a field I've designated in the Address File.

I'm pretty sure that the field I want to be automated needs to be a calculation field (not a lookup), I'm just not savy enough w/ the calculation sysntax to write it yet. Can anyone help this greying young man? Thanks much!