From Calais to Castelsagrat is about 575 miles and takes 10 hours to drive

A16 towards Boulogne (not Lille)

then A16 to Paris

then allow Satnav  to take you round the west side of Paris

eventually getting on A10 towards Orleans

A71 towards Vierzon

A20 towards Toulouse

then either:

leave A20 at Cahors Sud (junction 58) and go across country - saving miles and peage (but bendy)

or:

continue on A20 towards Toulouse

after Montauban take A62 towards Bordeaux

leave A62 for Moissac then on to to Castelsagrat

From Dieppe to Castelsagrat is about 480 miles and takes 9 hours to drive (ask for details)