My 24' round AG pool holds about 11,000 gallons, with an average water depth of 42 inches. If I loose approximately 1/2 inch per day to evaporation, that figures to be somewhere near 130 gallons a day as a comparison. With Your pool being approximately twice as big, 300 gallons/day may not be too far off. However, you will not see salt loss through evaporation. During your peak loss, you were double that amount so I'm glad to know you got it fixed.

What was the result of your first bucket test?

I don't have any experience with Salt Testers, but I can tell you that I get a different reading at just about every pool store in our area. Have you compared your unit against any pool stores?