Max trailer weight
Well, my trailer is about 6500 pounds and my truck is supposed to be able to pull 11000 pounds. It did fine on flat highway, but shifted down and was not able to maintain highway speed on any up grade (mild ones so far). So even though you'd think 60% of maximum would be fine, in this case I don't know if I'd want my rig anywhere near a 'mountain pass'. So the factor you are looking for appears to be dependant on the engine in your Tahoe. In the 6L class (350 to 380 ci), 60% seems like it would be problematical in mountainous terrain (at highway speeds, of course - at slow speeds you usually get rather more torque). If you have a good diesel engine or a much bigger gas engine (454 ci class), the factor would probably be significantly higher. I don't know as going over 90% would be wise even if you did not plan to go anywhere near mountains...
Engine torque and gear ratio are what determine if a truck can drag a trailer up any particular grade. You want to not only be able to do it, but not have the truck self destruct while doing it or shortly afterwards. Automatic transmissions are usually the weakest link in the drivetrain and tend to fail catastrophically. Therefore, unless you have a manual transmission, I'd make sure I had a transmission temperature guage and external cooler (heat is the usual cause of most transmission problems). An external filter could be useful as well (the unit I got contained a filter and temperature sensor in the same unit).