Found the algorithm I was looking for. If you can get the Zip+Lat+Long data into a table, you should be able to use this code.

lnLat = zip.lat
lnLng = zip.lng
lnMiles = (69.1*SQRT((lnLat-lnZiplat)^2+0.59*(lnLng-lnZipLng)^2))