Here's why a "rule of thumb" really doesn't work.
The GVWR of my trailer is a shade under 10k. If I am ultra conservative and use 25% to calculate pin weight I am at 2500#. However, I load to a GVW of only 8400# and my pin weight is only 15% empty or loaded. So my real pin weight when traveling is only 1260# or approximately half of the conservative guess.
A "guess" of 2500# would totally rule out a perfectly good trailer for me to tow. Likewise, if you guess wrong on the other direction you could be way beyond the capabilities of your TV.
I understand the dilemma. It is more critical for those with conservative tow vehicles. It is not always possible, realistic or desirable to just go get a bigger truck.
I spent considerable time studying brochures and making calculations before I bought my present trailer. It was time well spent. It has been a great combo and has served me well.