I had to design these once to hold up a conveyor in a farm building. I didn't have that much load on there, so I specified to tighten the 4 bolts to 70% of their ultimate load (as required in CSA S16 for slip-critical bolts) in an X pattern. I then used 75% of the value of the ultimate load in tension for my bolt to find my normal load on the post, just to take into account any creep or fatigue that could arise. With the normal load, the area of my plate and the coefficient of friction, I found how much I could withstand.
I guess you should also check the tube for a thin point load if you have a lot of load; this wasn't my case!
