I have 2 columns (i.e. origin & destination) & I want it to compare to data
in other part of spreadsheet to fill in miles between origin & destination.

For example: Chicago to Des Moines. I have Chicago in the origin column
and Des Moines in the destination column. I have the miles already
calculated for that run in another part of the spreadsheet, so every time I
fill in the orgin as Chicago & Destination as Des Moines, I want the correct
miles to fill in the mileage column.