this is how we install tennis net post sleeves. there are many ways to do this, some people use the flare out method, some people like us use the elephant foot or bell bottom method. as a guy who revamps courts for a living I've dug up at lot of post to replace them . I've had them with 500 pounds balls on the end of them . it's not how big your ball of concrete is its the shape that's going to hold it in place . We use an auger and we shoot in all different directions so it looks like a bell oran elephant foot. meaning the side opposite the net side you dig out like 12 inches this helps the post not kick in from tensionof cable. the earth above the area is going to act in a way that will push back against the tension....that happens once you get to your desired depth. at the bottom of your hole your going to have a gravel type base something that will drain any water that makes it into the sleeve.