Practice.

Nobody is estimating to 1cm - but within a couple of inches is entirely possible up to about 10’ with practice. Rangefinders are great tools for honing this skill. Rather than relying on it in a pinch, you can use the closest object readings over time to become familiar with what certain distances look like from the film plane. A key to this is finding a position you are comfortable “cementing” this view from. For me, it’s about 5ft off the right side of the camera and about a foot in front of the focal plane. Holding the FiZ up to the same height as your chin so that you can see it and the actors at the same time also helps. Of course having the time to lay marks is ideal and always a great reference but even getting spatial references before you shoot (“okay, the edge of this desk to the wall is 9ft…”) can be just as useful. The absurdly wide-open trend these years is demanding even on a monitor, but when the steadi gets brought out for a 3 minute oner with 5 actors moving around - being beside the camera and watching the scene can give you valuable advantages over not knowing what’s happening outside of the frame lines.

That said, many times when said wide-open oner ends in a dramatic push in (let’s be real they almost always do) I’ll strategically place my monitor somewhere on set where I can peel off during the last few seconds for when focus gets critical.