However, we have demonstrated that even the calculated fT values derived from the prevailing equations, based on linear law-of-mass action models or empiric equations, differ systematically from free testosterone measured by equilibrium dialysis by as much as 40%.
Perhaps I am misunderstanding this, but it sounds like there is 60% accuracy in most methods, if measurements can differ as much as +/-40%.
You’re the expert here though, so I’ll defer to you.