CuriosGeorge, again, your page says. I'm assuming you built it ... maybe not.
"Operate vehicle in compliance with all OEM specifications and LEGAL limits."
Apparently, you are a member of the weight police, but if you've followed the discussions here enough, you would know that your statement isn't considered fact by everyone.
Simply changing the tires to a slightly higher rated tire changes the OEM specs. OEM specs are in conflict because axle limits and GVWR don't add up. Which OEM do you want people to follow. Should they follow them some of the time? Are any improvements ok? What if I put a higher rated axle that goes against the GVWR tag? You can upgrade your brakes and then be out of OEM specs. Is it bad to improve your brakes out of OEM specs? How about HP, is it bad to make your engine more powerful? Not so simple, huh?
Furthermore, in some locals, legal is what you want to pay for. In effect, you can pay money to rate your vehicle higher than the OEM. Again, your website has opinion since legal is not black/white. The vehicle would be safe even if was illegal.
There are definitely dealers/manufacturers who will let you weight their TCs. But, if they didn't, how could you trust that their dry weights were actually right. There are lots of documented cases that the dry weight on the tag is hundreds of lbs off. Again, your web page is an oversimplification.
Enough of that ... a constructive comment about your calculator. I have an actual weight of my rig loaded. Given the actual load, tire size, axle ratings, your calculator should be able to tell me I'm safe. Alas, your calculator has no idea. It's looking for my trucks empty weight. Ok, if you intend your calculator to be only a prepurchase tool and a "guess" if someone who agrees with your guidelines might be safe or not. It's a guess until the camper is on the truck and loaded. Your webpage is providing a guess as long as the person looking at it agrees with your definitions ... again, your opinion. Totally ok, but it doesn't change that it's an opinion and a guess.