Gross Margin is the calculated difference (margin) between the value of the OUTPUT of an enterprise less the variable costs (INPUT expenses) used to produce that output. It is a reliable but simple gauge of how well an individual/enterprise is working in financial terms. GMC is a tool for farm recording that provides; Crop and Livestock Workbooks and spreadsheets (in MS Excel format) on a quarterly and annual basis.
Worksheets are available; at http://gmcalc.techtalk-international.com/.