Tom,
One acre, one inch deep, is 27,000 gallons. Since you have a 1/2 acre pond, one inch of water would be approximately 13,500 gallons. If you have a well that pumps 20 gpm, you would have 1,200 gallons per hour, or 28,800 gallons per day. That means a half acre pond would rise more than 2 inches per day, a foot in less than a week.
Essentially, you will have enough water to keep the pond full all the time, if you want.
One word of caution, though. Learn about the aquifer where the water comes from.
I have seen well water do funny things to ponds...like be the main cause for muddy water due to an ionic imbalance of dissolved minerals. I've even seen water lacking in certain minerals as to need to be amended with gypsum or buffered with lime. If you can get a water analysis of that aquifer it would be helpful, especially if you do it before you drill the well. I've seen several cases where the landowner would have made a different decision regarding a well...if they had only known the issues in advance.
