Tracy,
We driven this route from Calais to Carnac - and from Dieppe you'll join the Autoroute a good way down from Calais.
Join the A29 - head in the direction Le Havre - cross the Seine using the Pont De Normandie - then follow the signs for Caen.
At Caen follow the signs for Rennes - then Lorient and you are within 15 minutes of Concarneau
I seem to remember than Calais to Carnac was over 8 hours, so I'm guessing that Dieppe to Concarneau will be 6 hours.
Robert