So... we have 42 hours out and 48 hours back. Can someone whose brain is still workable figure out where the extra 6 are distributed?
You can't work it out. At least the details of how it is worked out are not written down anywhere.
If you can find a scan of someone's Brevet card then you'll be able to work it out from there.
Here's one from 2015:
https://i28.photobucket.com/albums/c204/cvserina/88_zpsh5dm56ly.jpg but, of course, the times/distances will be different in 2019.
Finding mine from 2011 (can't be 100% accurate with these as photos are blurred or timestamps obscured by stamps):
Start 0km 21/8 18:00
Mortagne 140km 21/8 22:27 au 22/8 03:20
Villaines-La-Juhel 221km 22/8 0:52 au 22/8 08:44
Fougeres 310km 22/8 3:57 au 22/8 14:40
Tinteniac 364km 22/8 5:45 au 22/8 18:38
Loudeac 449km 22/8 8:49 au 23/8 01:08
St Nicolas Du Pelem 493km 22/8 10:23 au 23:08 04:31
Carhaix-Plouguer 525km 22/8 11:32 au 23/8 06:59
Brest 618km 22/8 15:08 au 23/8 13:19
Carhaix-Plouguer 703km 22/8 18:24 au 23/8 19:51
St-N-d-P 736km 22/8 19:47 au 23/8 22:24
Loudeac 782km 22/8 21:42 au 24/8 01:56
Quedillac 841km 23/8 00:23 au 24/8 06:37
Tinteniac 867km 23/8 01:34 au 24/8 08:47
Fougeres 921km 23/8 04:01 au 24/8 13:04
Villaines-La-Juhel 1009km 23/8 08:01 au 24/8 20:12
Mortagne-Au-Perche 1090km 23/8 12:00 au 25/8 01:56
Dreux 1165km 23/8 1645 au 25/8 07:12
Finish 1230km 23/8 19:00 au 25/8 12:00