Well, there needs to be some breathing room- if the lens is at MFD, then you're not going to get anywhere because the AF might not lock on or be able to make a mistake.
I would recommend starting by shooting close to the minimum focusing distance, but far enough away that the whole lenscal unit is visible in the VF. This is so that you minimize the DOF and make any potential ff/bf problems as easy to discern as possible. You can also try the test and larger distances and see if any misalignments have an adverse effect in practice. With a lens as wide/slow as the 15mm, the DOF is very wide, so FF or BF is typically not going to be as much of a problem as it would with longer lenses like the 77mm or a 50mm.
Also, the reason their rule is off is that you can't generalize stuff like this in a linear fashion, so it's more like a rule of thumb. There are some lenses that focus closer than 10x their FL, while there are some that can't focus until you're 20-30x out
Adam
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