That's a good target size for a seasoned slinger. It might be a bit small for most though. Bill has some good advice. Can't go wrong with the hit and step approach.
One additional option would be to increase target size to where you are hitting it half the time at 10 meters. Then proceed with the hit a few take a step back, miss a few take a step forward. Once you are hitting half of your shots at 20 it's time to decrease target size 25-50% and start over. By the time you've done this 2 or 3 times you will go back to your original target and it will be ridiculously easy to hit even at 30 meters. (Not that small one mind you, the 50% at 10 meters. That small one will never be *easy* to hit at longer distances.
)
Though, you will have a hard time making progress to the higher levels of accuracy with poor ammo.
The biggest benefit you have right there is a natural backstop. Which cant can't be overstated for slinging. That lets you make some really nice ammo and not lose it right away. And a nice set of ammo will make a big difference.