I guess I just answered some of my own questions. My MicaSense camera has an online calculator... where you can put in desired flight speed (10 m/s), altitude and overlap. It then gives you (among other things) the track spacing: in my case 22 meters.
I just created a test mission in DD on my desktop and put in 100 m altitude and default overlap. Using Google Earth, I measured the width of the field and divided it by 22, which gave me the number of tracks. I used this to manipulate the overlap percentage in DD, which needed to be 88%.
I never used the simulation feature before, but I gave it a try just now, and this yielded exactly what I needed: the flight speed. By coincidence, this was the same as what I entered in the MicaSense calculator: 10 m/s. So now the only question remaining (@chasemgray): is this simulation speed the same as the real speed, when you actually fly the mission? Thanks!