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?
Extra 6? The time limits are 84 hours for the fast guys and 90 (42+48) for the rest.
I think this is based on the confusion many have over how ACP do cut off times for controls. Ignoring the special cases for controls in the first 60km, a control from 60-600km has a speed of 15kph. Meaning that if the Brest control was on the dot of 600. Then the cut off would be exactly 40 hours after starting.
That's the point, they don't apply this rule like this. For 2011 it was on the dot of 15kph for just the first 310km (to Fougeres) and then it drops slowly until Brest where it goes up again and then is up/down for each leg on the way back (but still less than 15kph).
(control, distance, open time close time, cumulative min average speed, leg min average speed)
Start 0 0:00:00 0:00:00
Mort 140 4:27:00 9:20:00 15.00 15.00
Vil 221 6:52:00 14:44:00 15.00 15.00
Foug 310 9:57:00 20:40:00 15.00 15.00
Tint 364 11:45:00 24:38:00 14.75 13.58
Loud 449 14:49:00 31:08:00 14.42 13.04
StNdP 493 16:23:00 34:31:00 14.25 13.00
Carhaix 525 17:32:00 36:59:00 14.17 12.96
Brest 618 21:08:00 43:19:00 14.25 14.67
Carhaix 703 24:24:00 49:51:00 14.08 13.00
StNdP 736 25:47:00 52:24:00 14.04 12.92
Loud 782 27:42:00 55:56:00 13.96 13.00
Qued 841 30:23:00 60:37:00 13.83 12.58
Tint 867 31:34:00 62:47:00 13.79 12.00
Foug 921 34:01:00 67:04:00 13.71 12.58
Vil 1009 38:01:00 74:12:00 13.58 12.33
Mort 1090 42:00:00 79:56:00 13.63 14.13
Dreux 1165 46:45:00 85:12:00 13.67 14.21
Fin 1230 49:00:00 90:00:00 13.67 13.54
(You'll see the overall average speed doesn't drop steadily and it also increased a tiny bit in the last few stages).
All one can do is keep an eye on the brevet card for when the next control closes (adjusting for your own start time) and make sure you arrive with enough time in hand (especially if you want/need a sleep there). If you've got multiple days to go then you need to build up a time buffer each day so that you can get whatever sleep you need (everyone is individual).