Whelp, I performed the test as described, and got good news/bad news.Lots of info in this thread: https://www.mu-43.com/threads/em1-micro-focus-adjustment-why-and-how-to-perform.95155/
As a quick test put 3 bottles close together, in a diagonal line. Put them at a reasonable distance away. Focus on the centre one and take a few images using C-AF and see which bottle is the sharpest.
If the nearest one is sharpest then it is front focusing. If it's the back one then it is back focusing. Adjust the microadjust value by PLUS for front focus (or MINUS for back focus) and then it is trial and error until you get results you are happy with. You can easily set it to 0 again if you are not happy with your changes.
If you save a setting it will only apply to that lens / converter combination. If you use the lens on its own that setting will be switched off. If you want to use the converter on another lens then you would have to adjust again for that lens.
Good news? Both my 300 and my 300 with the tele are dead on. No adjustments needed.
The bad news? See the’good news‘ section. I suppose this means my copy of the tele is so so at best. I was really hoping this would be a magic fix. Oh well. It was worth a shot.