[Cellarman]

The simplest way is to photograph some graph paper at different settings on your camera, measuring the distance from the front element. Do a set for each zoom setting, say at 20mm intervals on the zoom focal length and vary the focus setting on the zoom from infinity to closest focus by a fixed amount also. Record the data: zoom setting, distance, focus setting and magnification then draw a graph of zoom setting against magnification, or rather a series of graphs depending on zoom settings.