I believe this code willl work:
"http://maps.google.com/maps?f=d&source=s_d&saddr=" & YourAddressFromField &
"&daddr=" & YourAddressToField &
I did some tests and the web viewer and google maps adjusted accordingly when i changed either address. I got the code off google maps by entering some addresses for directions, then clicked on SEND or SHARE (can't remember) and the URL was right there to copy.
Use the above url code in a calculation field, then point your web browser to that calculation field.
That was fast, and very accurate! Thank you very much!