My system will be 3 tubes long. The third tube will be 72" long, 12" of which is overlap into the second tube. I choose this 12" length arbitrarily based on existing designs, so it is NOT critical.
Yes, you are going in the right direction. I like the telescoping tube design for storage and handling reasons. You will need to make sure there is plenty of contact bearing area between the tubes as that the fits are not so loose to cause point loading resulting in an over stressed location.
BTW, the stress at "x" or "Stress at a specific point" is interesting; however the number that you should pay attention to is the "stress at the support". That load (stress) will be the maximum loading realized by the loading configuration.
Also, don't forget that you will need to determine the loading carrying capacity of the connection points. I'm assuming that you are going to use a few welds as well as a couple of pivot shear pins to support the loads. These will need to be understood as well.
See:
http://www.engineersedge.com/weld_design_menu.shtml