I don't know of a formula for that one. Again, if it is a 36" hub into a telco closet or between floors I would be inclined to think I could carefully load that up to 80% or so without damage. If it is a 100' long run with 2 90 degree corners in it, totally different story.
In the past, I have installed (while being told what to do) wire, pulled so hard that wire actually suck back inside the jacket 6-8 inches. My guess is that was about 80-100 pounds of pull. Actually terminated, scanned and tested it to 5e standards no problem. I certainly wouldnt recommend it, nor do I allow it on my installations, but that wire can take quite a pull. Of course, if it doesn't test out, you have to pull the whole mess out and do it over, so generally it just isnt worth the risk. As we ask more and more from our copper cable, we will have to be more carefull in installing it so that it will pass higher standards.
I find it best to pull those larger bundles as one pull, distributing the load/weight over the entire bundle if possible. If it is pulled in one or two at a time, I would definately soap the heck out of it and stay at a lower fill percentage. Each wire going in drags across other wires and can often burn and damage the existing wire. We try to pull to the telco room and leave slack, then pull the entire bundle in through the 4" feed conduits where possible to avoid the chance of burning the existing wires.
Again the answer depends on the specific installation and application, and whether you are designing this to be a standards compliant installation (i.e. spec another conduit so you can keep the fill ratio compliant) or if you are installing in an already built structure and you need to determine how many you can safely put in existing raceways.
It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD