jeff, thats an expensive data set but the fact the pond stabilizes half way, w/ 20 gpm going in 24/7 (28,800 gallons/day) tells you how big yer leak is. my guess is that when the water is higher assuming you get occassional rain run-off which exceeds 28.8k gallons per day, the head pressure on the leaking area increases, and thus the leak "production" increases accordingly until the water "equilibrates" back at half pool.....this is how my pond used to be, and without supplemental water, dried up completely in a matter of a month or so.
my fix was a straightforward dig out, clay line all suspect areas, let it refill and watch, i now hold water year round although there is some seepage at full pool (due to max hydraulic pressures at full pool), and then once its down 2 to 3 feet, summer evaporation takes care of the remaining 3 to 4 feet it drops (6 to 7 feet total over the summer).
good luck, and i'm sure mr. lusk will have important feedback for you.
