The real value of the Zone System is being able to visualize a scene and exposing to reproduce your visualizing when printing. I think Fred Pickering in his Zone VI Workbook does a good job of explaining in a straight forward manner the Zone and how to test for personal ISO. For roll film is Carson Graves' Zone System for 35mm Photographers. He shows how to test for personal ISO without needing a desnistomer. For a standard record shot an internal meter with a gray card or incident meter works for even the most tricky lighting. I have a number of meters, but last few years I when shooting large formate I use the internal meter in my Sigma SD 15, matrix mode, with a strofome cup to fit over a 17 to 55 zoom, in Spot mode and a 70 to 300 reads less than 5% of a scene, it is not much larger than my Soilgiar Spotmeter.