FoCal does multiple tests for focus accuracy, then arrives at a recommended adjustment.

How useful that will be to you depends to a large degree on the camera you're using.

Not all cameras have a micro focus adjustment feature. For example, among the current Canon models only the 80D and higher models have it... none of the "Rebel" series have it.

Older Canon cameras had means of adjusting up to 20 lenses and only doing a single adjustment per lens. But, worse for some of us, they weren't "lens specific". MFA made lens MODEL adjustments. For example, if you had two Canon 50mm f/1.4 lenses and determined an adjustment required for one of them, it would be applied to both. Now, most people don't have multiple copies of a lens so this is only a problem for a few of us (I currently have two 300mm f/4 and two 28-135s... have had multiple copies of other lenses in the past). What's a correct adjustment for one lens might be totally wrong for another copy of the same lens model. It also wasn't ideal for zooms, since only a single adjustment was possible. You often had to compromise with zooms.

Newer Canon DSLRs really improved on this. The ones with MFA now can be programmed with fine-tuning adjustments for up to 40 lenses, have means of making two adjustments with zoom lenses, and the adjustments are lens-specific (MFA refers to the lens' serial number, to distinguish between and allow different adjustments for multiple copies of any particular lens model).

FoCal recommends that the target be set up at a distance that's 50X the lens focal length.... so for a 50mm lens it's roughly eight feet away (50 x 50 = 2500 and there are just over 25mm per inch, so call it 100 inches, which is 8.3 feet). The problem comes when fine tuning large telephotos. Based on that recommendation, a 400mm lens would require the target be set up 20,000 millimeters... or approx. 800 inches... or roughly 66 feet away! If that's a problem, it may be possible to use 25X the lens focal length... but still requires a lot of space, with the target over 30 feet away.

You can fine tune with MFA without a software like FoCal. What Canon recommends doing is simply focus on a target at a moderate distance using the in-viewfinder focusing array... then switch to Live View and focus on the same target. If the two don't agree.. if the lens changes focus at all with Live View... then there is some adjustment needed. You can experiment with + or - adjustments until repeated testing shows no difference between "normal" and Live View focusing. This is because Live View uses the image sensor itself to focus... so needs no MFA at all. If the "normal" focus agrees with that, it's accurate too.

Or, you could just switch to a mirrorless camera and never have to worry about any of this. That's because mirrorless camera's focus using sensors embedded directly in the imaging sensor itself (just like Live View), so there's never any need to adjust them at all.

I think I could get sharper images, from my rig and am contemplating trying FoCal. I know some of you have and use this, to further dial-in your glass. Can anyone speak to how and if this can be accomplished with a telephoto lens? I have read the information that I can find on their website, but am looking for more "real-world" feedback. I have already sent lens and body to the maker of the lens, but am trying to squeeze a bit more out of my combo. If you are one who feels like it is a waste of time and who would chastise the use of anything but a Nikon lens on my Nikon, please do not bother. Thanks!

Regardless what others will write here, here are the facts.
1. you can only adjust your lens for one distance, if you are going to adjust your lens, adjust it for a medium distance. Remember, once you have calibrated your lens, it is only good for that distance. PERIOD, end of story.
2. If you take most of your photo's with a telephoto at maximum distance, you will not need to calibrate your lens.
3. If you usually stop down your lens 3 or 4 stops, you will not need to calibrate your lens.
4. I have owned more than 20 Nikon AF lenses and enlarge my show prints to 20X30, I have NEVER had to calibrate ONE of my lenses to get perfect focus.

Bill, the OP specifically asked us not to indulge in this argument (he’s probably seen it ad nauseum before), but since you’ve initiated the discussion, let me make a couple of corrections:

1) while you can only cal at one distance, that doesn’t mean it isn’t very close at other distances. Despite your assertion, I have actually tested this and you can see the results in the link I posted above. If you have actually tested and can prove differently, please post the evidence.

2) contrary to your assertion, because long teles have a narrow DOF, you cannot accurately make this blanket statement. For example, a 600mm lens on a FF at 50’ distance at f5.6 has a DOF of just 0.69 feet (~8””), so accurate focus is critical

3) I agree with you on this - if you shoot stopped way down, the increased DOF may “hide” AF errors

4) since you have never calibrated them, you have no idea as to whether they were focusing accurately and could have been improved or not.

By “telephoto” I assume you mean “zoom”. (A single focal length telephoto such as 200m is treated like any
other single FL lens.)
So, a zoom does present a problem. What I did was did was run the calibration three times - at minimum zoom, max zoom and mid point and record the results. Then I picked calibration adjustment that minimized the overall error -ie a compromise solution. An alternative solution in certain cases would be if there is a a particular focal length you use most of the time. Keep depth of field in mind - if you routinely stop down quite a bit the DOF may compensate for the focus error