The key to runners, I feel, is the "full-out" Release, always full-out to the end of the arm, same speed and force (about 70-75% of max.). When your Release is totally predictable, then all you're judging is the angle, the height. That simplifies it down, so it should be easily learned. Of course, you shoot on the way up, from a good percent of UpForce, to give you stability. If in very close, then you can have hang time before releasing the ball, but don't lose it all.
Check it out. Is your Release full-out, the same each time, or is it varying? The judgment of the angle (height) is instinctive. You'll know what is appropriate with practice and experience and trust.
I'm even considering adding "Runners" to my coaching of kids, even beginners, as it will show them how accomplished they are becoming, even with this supposedly very difficult shot. I've had coaches tell me, "Oh, you should never tell the kids to shoot runners. They're too difficult." Well, maybe, but maybe not.