I agree, or one could blindly be under the impression that because it's good at 24 and 70, or in the example he gives 70 and 200mm, then it would be good everywhere along the focal length. That leaves me with my original concerns. However, I will retest my 24-70mm as I conducted it a f2.8. That is because of other lens checks I've done with e.g. LensCalc and the camera at 45 deg. to a card DOF target. I should have thought that through as f8 is around the sweet spots of most lenses.
It is misleading because the FocusTune manual Page 3 states " 2.............Shoot wide open (for example f1.4)