How do you calculate evaporation for a lake using data from a class a pan?

The conversion from class a pan evaporation to lake evaporation is done by a pan to lake coefficient. The coefficient various from region to region, but is generally from 0.6 to 0.8 and more specifically from 0.7 to 0.75. I would recommend using 0.7 as a ratio. Once you establish the ratio you simply take the class a pan evaporation value and multiply by the ratio. For example, if you have 50 inches of class a pan evaporation then you would use the ratio of 0.7 to estimate 35 inches (0.7*50) of lake evaporation.