The 'Ready Reckoner' is a model which performs a simple, site-specific economic assessment of the viability of evaporation mitigation systems. The user enters appropriate data to customise the Ready Reckoner' to their site. The Ready Reckoner' returns the volume of water saved (in ML) and the cost of the evaporation mitigation system used to save this water ($/ML/year). The Ready Reckoner' was developed as a Microsoft Excel spreadsheet for ease of use and portability. This reference manual was developed to assist the user in the operation of the 'Ready Reckoner'.