Use your natural stereoscopic vision and stereoscopic hearing together with the tape measure to become aware of relative distances. Carry a 1" width Metal tape measure for close measurements from the camera. Carry a cloth tape measure to walk out to the subject from the camera for slightly longer distances to 33’ or so. Measure at the appropriate time to not interrupt the flow of the set. Make T-marks for the talent. If you use a laser or sonar system use your own depth perception to verify that the readouts are logical and correct. Eventually you will have memorized the Samcine Calculator, or ASC Manual depth of field tables or the pCam app DOF. They will let you know what you can get away with. You can achieve a Focus Zen by learning to not rely solely on a monitor. The easiest way to visualize DOF is to know the size of your shot (Field of View). If it's a close-up focus depth is very narrow, pay attention. Anticipate leaning by actors. If the shot is wide, like two people on horseback head to hoof, you will have more depth. And the talent marks will be in shot, so make them environmentally hidden or within proximity. It doesn't matter what lens it is, if you have about the same size shot at the same aperture it doesn't matter the focal length, the DOF is about the same. Although on a long lens it is easier to see it fall off. Check me on your DOF calculator and this will be verified. Of course wide open stops (T1.3) are always more critical than a deep stop (T16). Feel free to privately and diplomatically suggest to the DP that they use a deeper stop for very tight closeups or macro inserts. They have an interest in keeping important actors in focus too. Eyeball focus for lenses over 150mm and distances over about 50 feet. Your stereoscopic depth perception is less accurate far away. Do prep your lenses carefully. Know that the markings are correct. If they are not, and other lenses are not available, put paper adhesive tape or chart tape on the lens barrel and make your own distance markings based on a tape measure reading. If you get a lens marked in meters and you use feet (or vice versa), make your own scale with the accuracy that you need. Mark both sides of the lens. Of course, make sure the back focus or lens mount shimming is correct. On a film camera you might test to make sure the mirror is not out of alignment. This causes the eyeball focus to not agree with the focus at the film plane. This is rare, but happens. Check that the infinity mark is in fact focused to infinity. Some Panavision lenses focus beyond infinity. Establish where infinity is on these lenses. Check that the zoom holds focus through the zoom range. Check that the iris is functioning correctly while you’re at it. Good luck. Achieve Focus Zen and sleep well at night.