Do not think you will find any statistics on that distance. Most pros manage their shots better where they are hitting full shots into the green. That would be a chip or pitch shot and their accuracy rate would be pretty high, most would be looking to make it. 20 yards equates to 60 feet and I would say, depending on the lie, edge of green, a few feet off the green, in the high nineties. Your percentage after thousands and thousands of golf balls could be in the high nineties as well. That is what it takes for the pros. Keep up the practice. A shot like that would normally be into a green and practicing like this would not have the same affect. This gives you only distance and target practice, not how the ball would react, but a range is not always available. =(

Nice. My suggestion would be to not look at a target number to be satisfied. Go for 100% and be the best you can be.

. My only criticism is you are hitting off of a mat which for me gives false confidence

How many do you really hit bad off a mat ..very few

I never hit our range when the mats are in use

Unfortunately, due the number of people using our range, we HAVE to use the matts Monday thru Thursday. Our courses entertains over 65,000 rounds per year. So that is a lot of folks hitting balls, plus those who just come by to hit balls, the range cannot tolerate that much usage seven days a week. Matts have their purpose at ranges. It takes such a long time for them to recoup with the amount of usage when some do not practice with a purpose, go just to bang balls, and leave the range looking like a newly plowed field. With the respect to all those who take care of our ranges. Guess it is all about choices. Practice is practice. It is how you use it.

Not a statistic, but I once watched Steve Stricker practicing at his home course at a similar targets, from 20-40 yards. Deflated my ego… Let’s go with Dale’s suggestion of consistent striking and as close to 100% as you can get. ;-)

One additional note. Aside from the fact that I only hit perfect shots from a mat…. Your collection of balls indicates a wide variety of balls. Planning on improving your short game, then you need to practice with the same ball.
Most amateurs don’t practice chipping/pitching enough so even with the same ball, they just hope for a good result. If you are going to hit hundreds of ball to your target, then your gamer and practice ball need to be the same. You can’t control launch, spin, and roll-out letting your next ball be the last one you found.
These type of shots are best controlled with a tour grade urethane cover. But there is still benefit to know exactly how a Tour Soft, Velocity, or TruFeel so long as you use it consistently in your practice.
In another answer, I mentioned Steve Stricker practicing. As a club member, I was using yellow NXT Tour S range balls - standard fare. Steve had his supply of white Pro-V1 balls. The range crew separated out the Pro-Vs after clearing the range. Practice well.