If you have a Blackmagic camera with an EF mount - Pocket 6K, 6K Pro, or Ursa Mini Pro - setting focus marks isn't possible. This is because on the EF mount cameras, we don't have any information on absolute focus position.
On the Pocket 4K we can say, 'Hey lens, I want you to go to 75% of your total focus distance.' And away it goes.
On the EF mount cameras, we can only say, 'Hey lens, I want you to focus a little closer (or further) than you currently are.' With no reference point to where it was focussed before, or where it is focussed now.
'Ah!' I hear you say, 'but I'm using an EF mount camera and I see the lens' focus distance right there on the screen.'
This is true, but this is only a very rough estimate, reported by the lens, and not something we can use to pull focus in any smooth, repeatable way.
Hope that makes sense!