You could use B and N to set up the work area in your main comp that you need for the duration of your subcomps. Then right click in the work area and select “Trim Comp to Work Area”. Press Cntl+K to find the duration, hit cancel and Cntl+Z to restore your Main Comp back to it’s original duration. More button pushing, but less math.
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.
I know this is a slightly old thread, but FYI, when you click on the work area bar, in the info panel there is a display that tells you the current duration and the start and end timecode of the work area.