I always make sure I know the distance to the end of the eyebrow or mattebox (often 40 cm) as an actual reference point. Then I visualize this distance 4 times in front of the camera (at 80, 120 and 160 cm) as virtual reference points. So I have 4 reference points that are especially handy when shooting handheld. The next reference point is when I stretch my arms completely (190 cm) (so the split second thought should be: when I put one hand next to the gate, can I touch the actors eye?). If you do this often, these reference points automaticly pop up when you stand next to a camera. (It's also kinda zen...)
When someone is sitting in a chair make sure you also measure the front of the seat of the chair as a reference.
Furthermore, one wise focus puller ones told me that focus is not a point but a range. So when you estimate distance don't think: it's 23.853 feet, but think: it's more than 22 and less than 25 feet. And when you get get more experienced you know it's more than 23 and less then 24 feet. So you learn to narrow it down. This is also very usefull when shooting handheld.
Steven ;)
