Reikan FoCal looks good - anyone used it?
Yeah I recommend it for any lens that has a wide aperture. If anything you can see how consistent the AF is. The software is automated and it does repeated tests and then averages it out. What's interesting is how the focus at these wide apertures varies - click +10, click again +6 and so on. So that means at any given time, even with a calibrated lens you're not gonna be hitting the target with 100% accuracy every time. All your doing is increasing that probability or getting it closer to accurate. Some lenses may only be slightly off in which case you might get away with leaving it at zero. But some will clearly be off when you see Reikan in action.
Forgot to mention it also depends a lot on your typical shooting distance. The further the subject is from the lens the more likely it is to be in sharp focus as the dof increases. So calibrate the lens at your working distance.