Unfortunately, they are all wrong. If your design temp is 85F, then the system should be running constantly when it's a sunny 85F afternoon or warmer. A 5 ton will still be cycling on and off. You don't design for 100F just because once every few years for a couple hours it might get that hot.
That's like buying a minivan to commute to work every day because of the off chance that your wife's van is being repaired so you need a spare. That's what rentals are for. Or buyign a pick-up because once or twice a year you go to Lowes and buy a bunch of lumber. That's why you have that stuff delivered. You get HVAC systems... and vehciles... for what their primary design or what you need them for is 90-95% of the time... not for the 5%. I'm not saying yu only buy practical cars or equipment. You can pruchase featurs based on comfort or preference. If you really love how that minivan drives and rides, than commute in that. Or pick a motocycle, corvette or BMW.
I have a 3200sqft, 2 story, 86y/o old home, I'm in SE Iowa with design temps around 93F, with high ceilings and I don't even need 5 tons. About 4 tons total. And I can cinfirm that based on run time this summer in design conditions. Although the contractor that the previous owners used must have been trained like that one... they installed 7 tons total capacity.