What is the optimum player position respective to mob grinder?

I've noticed a bunch of rules regarding monster behavior relative to player position: they don't spawn near the player, spawn then despawn far from player, spawn and stay somewhere in between, move in certain distance, don't move at a different distance... generally, a mess of rules I can't quite tackle. So, I'm building a mob grinder. From what I understand, if I'm too close monsters won't spawn, if I'm too far they will despawn and if I'm not near enough they won't move, won't fall in the flow and get carried to the collection zone.

So, where should my character be, respective to the designated mob spawn area of the grinder for optimal yield?

Mobs won't spawn closer than 24 blocks. They won't move around when farther than 32 blocks. Those are the only rules you need to know for a grinder that relies on mobs walking around.

The inference is straightforward: you want your collection spot to be 24 metres away from the nearest wall of the grinder, and you want the far wall of the grinder to be 33 blocks away.

Or put another way, you want your grinder to be in the 8-metre-thick ring or shell around the player where mobs will spawn and move. In this illustration of the spawning distance rules, this thin shell of viable spawning area is represented by the light blue ring:

This implies that the optimal grinder is shaped like a ring or a ball surrounding the player. (Note that these distances are true radii, so if the grinder is above you in need to be closer than 24 horizontal metres!) Construction considerations make such a design less than optimal though, so a nice compromise is one to four towers that each fully enclose a section of the useful spawning shell; even if some of the floor space is "dead" sometimes, the less-perfect design allows you to move around in your collection zone a bit more and still have the grinder(s) work well.

This is wrong. There is not only an eight block gap where mobs can spawn and move around. You can make many farms where the mob will spawn and move to either try to get a turtle egg or kill another mob. It is clearly just not an eight block gap.