[Brightcolours]

Two things,

 

1) Pretty sure some Canon DSLR had shims, no idea on 6D though.

2) Even if your focusing screen plane is perfectly aligned to the image sensor, along with the mirror, if your particular lens suffers from back/front focus with your particular camera, you can't compensate for that electronically. Which means if your Eg-S screen shows that you nailed the focus, you actually haven't. That's why I said what I said.

 

I had a 5D like that and figured out it would be worthless getting an Eg-S type focusing scren for that camera, I simply learned to rely on the focus confirmation beep instead. The lens with the massive front focus was a m42 mount Carl Zeiss, so I got an adapter with the EMF chip which let me dial in the required front focus microadjustment setting so that the beep would be accurate.

It seems that you have no idea what you are talking about? A lens can not front or back focus when it' projected image is in focus on the sensor.