I estimate the magnification ratio or determine it precisely by putting a ruler at the plane of focus and comparing the field of view to the width of the frame, then use a chart to determine exposure factor based on the magnification ratio. It works the same way for every format, independent of the lens. The principle is the same as with the QuickDisk. Somewhere here I posted an MS Word doc file that you can download with a magnification/exposure table in small print so it can be taped to the groundglass frame of your camera.