day 18,
esshup's calculations look correct to me.
[esshup, did you use an August evaporation rate of 4"/month? You might edit your post to indicate that rate. Then someone in an area with a rate of 2"/month can easily adapt your calculations without performing every step themselves.]
Also consider that the evaporation losses (in gallons) will go down slightly during drought periods as the surface area of your pond gets smaller.
The necessary water productivity of a well (and the corresponding electricity bill) will be much higher to keep your pond full compared to maintaining it at some smaller base level just to make it through to the other side of a drought period.
