For the most accurate estimates you can use the HERE API or Google Maps API to get estimates including traffic delays, mode of transport etc. It costs money per search.
Otherwise you could do an estimate based on the point to point distance which is easy to calculate using two lat/long pairs. FileMaker Custom Function:DistanceBetweenPoints ( Lat1; Lon1; Lat2; Lon2; Units )
Or you could create a point to point table and manually enter.
I put a more detailed response to the corresponding thread on FMForums, but I feel the need to have one point be more visible in this thread: