In the summer I probably lose 150 gallons a day or so to evaporation
with the level falling about 1/4" per day. If it falls below the skimmer
level the pump will such air because the bottom drain is no longer
functional. The problem is when I go out of town for work or on vacation
and no one monitors the pool level.

Yesterday I replaced the manual fill valve with a 24VAC valve and hooked
it to my sprinkler controller so I can add water automatically, but
there is no feedback as to the water level. I'm trying different daily
fill times to find one that maintains the water level relatively
constant, but it's inexact and depends on the weather.

I want to construct some sort of level detector that will cut off the
automatic fill valve if the level gets too high, or I could disconnect
the timer and hook the level detector relay directly to the valve.

I was thinking of this:

"http://www.ingramproducts.com/Pumps_Level_Controls-24_VAC_Multi_function_Moisture_Sensing_Relay.html"

and this:

"http://www.ingramproducts.com/Pumps_Level_Controls-Custom_Sensor_Assemblies_for_Liquid_Level_Control_ 1.html"

In the winter and spring I often have the opposite problem, the pool
overflows from rain, and I'd like to also do something that opens the
pump's drain valve diverter and removes water until the level falls to
the proper level.