No matter what you do, there will always be the difference between theoretical and practical performance.
Manufacturers include a "safety factor" in all designs, signifying the practical application performance differences. What I have seen, more often than not, is somewhat incorrect extrapolation of performances along with sizing without testing those extrapolation values. Of course this is for a manufacturer to decide and warranty.
Of course salesman cannot see this or they will try to argue that engineers are oversizing equipment which is why there are warranties.
As long as manufacturers live up to warranties and performances, that's the end of the conversation between supplier and user.
Everyone else's opinion is a mute point.