Hello Surfer,
I do not condemn the third party web sites outright, they can and do provide some guidance, but I point out that you cannot trust them implisitly because they do contain some errors and the only way you can determine if their figures are correct is to compare them with the manufactures provided data, by which time you can do the calculation your self and be confident in the output.
Unless the data is supplied and installed electronically without human intervention, errors will almost certainly have crept in. I cannot possible believe that the site operator has been able to convince all the manufactures to collate their model data in a common format. I know that I have found at least two errors over the years when researching answers for people on this forum.
Like Lutz, I do not keep a list of the inaccuracies I come across, but look through this forum (and others) and you will find many instances where contibutors have used these sites and found errors in the data or models completely missed.
The calculations needed to work out towing ratio, (and heaven forbid the dreaded 85% and 7%) and other leagal limits is well withing the capabilities of a driver with a calculator, so these sites are not strictly neccessary.
Also as Lutz points out, most of these sites do not carry all the relevant information, such as max towball loads, GVM and GTM MAM so they only provide part of the story anyway.
It is the drivers respnsibility to check thier outfit is road legal, it would not be a legitimate defence to claim a web site or similar service said it was ok.