An article in Photo Techniques (Mar/Apr 96) on view camera focus used this metho d. Basically, you tape or glue a mm scale on your bed, and then focus on the nea rest object of interest, read the position on the mm scale, then focus on the fa rthest object of interest, and note its position on the mm scale, and then, usin g this focus spread (in mm), refer to a chart for optimum f stop to use, based o n line pairs per mm resolution. I photocopied the chart and taped it to the back of my camera for reference. I hope this helps.