Calculate the approximate beam diameter
For a circular beam hitting a surface square-on, estimate diameter with 2 × distance × tan(angle ÷ 2). Use the full beam angle in degrees and measure distance along the beam from fixture to target. The diameter uses the same units as the distance.
At a 10 m throw, a 5° beam covers approximately 0.87 m, a 15° beam 2.63 m and a 30° beam 5.36 m. To cover about 3 m from 12 m, the same calculation in reverse gives approximately 14.3°. That is a starting point for choosing a zoom setting. Oblique aiming stretches the footprint, and beams with asymmetric optics need separate horizontal and vertical coverage checks.
Compare angles measured at the same light level
Photometric charts can show both a beam angle measured at half the peak intensity and a field angle measured at one-tenth of peak. The second measurement normally describes a wider area. Check how the manufacturer defines each angle before comparing fixtures or putting the figure into a coverage calculation.
A calculated diameter describes one contour, so it does not tell you that the entire circle is evenly lit. If you are overlapping fixtures for a wash, read the intensity distribution across the beam and check where adjacent fixtures meet.
Compare centre lux at the intended throw
Lux measures illuminance at a surface. Compare readings for the same distance, colour, output mode and zoom where possible. A narrow white setting can have much higher centre lux than a wide saturated-colour setting on the same fixture.
If you only have one measurement, the inverse-square relationship can give a rough estimate in the far field with the same optics and output. For example, 2,000 lux at 5 m is approximately 500 lux at 10 m under those assumptions. Use the manufacturer's distance chart or a measurement at the intended throw when available.
Put the venue measurements in the equipment brief
Record target width, throw distance, surface orientation, desired overlap and the colours the show will use. Compare a narrow moving beam for aerial effects with a zoom wash for area coverage on the basis of that brief. During a demo, check the edges and the light level at the subject as well as the centre of the beam.