I think the most common application of the 80% rule is when people are not going to account for a worst case scenario or don't want to physically weigh their trailer, vehicle, tongue weight etc. but still want to be reasonably confident they won't be over any of their ratings. For you, you are assuming a worst case scenario where the tongue weight is 15%, trailer is at GVWR etc. and you are then adding 20% on top of that. The only harm in doing that is that you're severely limited your SUV choices, they will not be newer vehicles, and most of them are not very fuel efficient when not towing. A 3/4 Suburban/Yukon XL from 2000-2006 would just meet your requirements. Some of the later years of Excursions could likely handle them as well. Earlier years of Excursions the hitch was only rated for 1000 lbs of tongue weight I believe. I think that is about it. If you don't mind a pickup truck you have many more options, still generally going to require a 3/4 ton however.
I think if you're assuming a worst case scenario with the trailer weights and you know the actual weight you'll put in the vehicle in addition to the tongue weight you should be fairly safe just sticking to 100% of the vehicle ratings or if you want to play it safer go down to 90-95% but using 80% is driving you beyond what is necessary unless you plan to use this vehicle for towing only and don't plan to drive it much otherwise. If it is primarily a tow vehicle and noting else, then overkill isn't such a big deal beyond the upfront money to buy it. At least it isn't draining your wallet for gas every day.
On most suv's the limit ends up being the suv GVWR or tongue weight rating of the vehicle hitch, many of which are not replaceable hitches so you are stuck with what the manufacturer put on at the factory for a hitch. Good luck.