I finally dialed in the last of my Canon lens today, a 400MM F5.6, and it was the only one that required a major compensation. By itself -8 was needed and with the 1.4X converter in place -1 was correct. Most of my lenses were on with little or no correction, for example my 70-200MM F2.8 was -2 by itself and 0 with the 1.4X in place.
I was a little surprised with the 400MM to see how much difference there was in required compensation between the bare lens and with the 1.4X in use.
Rather than follow the exact Canon procedure I shot a series of 5 shots each with the compensation at -10, -5, 0, +5, and +10 with and without the converter in the first iteration of testing to get a basic idea of where the compensation needed to be set and then fine tuned it in the second go round. The EXIF data nicely shows the AF microadjust compensation level so you know exactly where it was set for each shot.
I am planning to try the 400MM for some of my daughter's daytime soccer games this spring. It was a little too long for a lot of the work on my older 1.3X crop 1D Mark II but seems to be a good match for the full frame 1DX Mark II. But I expect the 70-200MM 2.8, often with the 1.4X converter in place, will get a lot of use. I plan to pick up a 300MM F2.8 before the Fall indoor season if I like shooting the outdoor games with the 400MM prime. The indoor dome is pretty tight and I know a 400MM would be too long for many of the shots there. In any case going from shooting primarily with a zoom to a prime will take some adjustment on my part but it will be a good learning experience.