Height is a big factor, but another thing is the leg to torso ratio. That's why some people who are taller can state that it's not as much of a space issue for them.

I'm 5'11" and a bit, so I'm not nearly as tall as some of the giant Scandanavians on the forum. Still, I have a good bit of leg length. On a plane my knees are typically very close to the seat even when it's upright.

I view it similarly to when I'm in a car. I never just run the seat back if someone is behind me. I check first to see what their seating situation is, and we find a happy medium.

Honestly, it's never even occurred to me to lean my seat back on a plane because space is so tight, but maybe for a person who hasn't experienced the issue (because they're shorter) they just haven't thought about it.

Seems to me that the best rule is to simply ask/check about the seating situation of the person behind you. We're all traveling together so simply saying "I'm going to recline because I can, and it's not my fault if I hurt your knees." or "'I'm going to assume that you're about to bash my knees, and stop that." are both too presumptuous.

I do think it's rather telling of how personal space is shrinking on planes that someone felt the need to create this device, and that it is selling.

As tight as they've made space on planes today, the reclining option does just seem like a fight waiting to happen.