I still think that using the 30 year (some places 50 year) NOAA pan evaporation data provides a good approximation to answer your question Greg. Pan evaporation rates are measured at all large airports and in multiple locations in some counties. They also measure precipitation data. Lakes and ponds are said to experience approximately 70% of the pan evaporation. The data is provided as annual median and mean, as well as monthly median, mean, lowest recorded, maximum recorded and raw data is provided. Based on this and your estimation of the type of year you are having, you location vs, the measurement point, local humidity and temperature, etc, might help you estimate your particular situation.
If you want to set up your own pan evaporation tool, they provide a detailed description of the pan. It is a 4' diameter pan that is 1' deep I recall. Measurements are taken daily at a set time and water replaced as evaporated. In my case I am equidistance from 2 sites with very similar data. My assumption is that I am in line with he average of those two sites. It is good enough for my needs.
