Most homeowners don't care about the calcs on retrofit projects until it's too late and can't/won't justify the cost when another contractor is 1 or 5 G's less.
New is all on you, so the calc accuracy period is moot when their attorney calls.
Scope of work on retrofit work matters too. You propose to replace a furnace and they don't want to discuss other issues... or let's say you overlook something minor. As long as you weren't compensated for the work, it could be argued that it's not warrantied. Double the price and offer a money back/satisfaction guarantee on every project and hope they don't call?
Gross mis-sizing/application is obviously an entirely different situation that we have all seen, heard about, and dealt with. The guy that puts 5 tons of cooling on a 3 ton duct system probably has a tail light warranty.