You're absolutely right. I use the same calculator.
Last month, I put two new tires on the front of our coach. The original size is 295 75 R22.5 . The dealer almost installed Michelin 275 80 R22.5 . I insisted on the original size tire. He had Bridgestone R280s in the original size, so that's what I bought.
Using the calculator, I now realize that the dealer was correct. Having said that, I am very pleased with the Bridgestone R280 tires.
Jim & Kate
2016 Creekside 23RKS, 2014 Ram 1500 Eco Diesel
1995 Monaco Dynasty 36', 2005 Honda Pilot