We crossed the Reno in a boat.

This does not include the fight with Reno before his retreat, but from the time that Custer's command advanced and the fight with his command commenced.

The natural thing to do would have been to retrace his trail, join Reno by the shortest route, and then, united, have pushed the attack in person or, if then too late for successful attack, he could, in all likelihood, have extricated the command and made junction with Terry.