Selection of the area of irrigated cotton to plant is a perennial problem. Research by the Centre for Water Policy Research (1989) has shown that growers solve this on the basis of their total water allocation and expectations about the price of cotton. Hearn (1988) has shown how the yield of cotton changes in different years with different field allocations of irrigation water, using a cotton water balance model. However, this analysis is in tabular form and difficult to customise to suit individual situations with different land area, water supply, irrigation efficiency, soil water deficit for irrigation, maximum yield, cotton price and variable costs. In this paper, we show how a lookup table of the kind provided by Hearn can be re-formulated as a spreadsheet template in order to simplify the analysis of the underlying lookup table and provide a tool for decision support.