Using this map as a guide, do all agree that Martin was sent back at about the "H+30" position of Custer's squadrons? This model has Reno on the bluff top about 10 minutes later, and Benteen with Reno 5-10 minutes after that.
If you look at all the "H+40" positions, Benteen's force would be about at the Custer H-hr at that time, moving up at the canter.
This model has Yates just pulling back from Ford B as Reno and Benteen meet. I presume Martin is with Benteen? That would mean that Martin and Benteen meet Reno about 20-25 minutes after Custer had cut him loose.
It couldn't have taken Martin more than 10 minutes to find Benteen, presumably near where Custer left Reno's path to move north up the bluffs?
Here's the ford area labelled, for reference...(the two closest coulees to the river off the north face of Weir point I haven't labelled...some call one "western coulee," but I'm not sure which one)...