Looks like the best value is 18 for mobs.
Any higher and you are out of its aggro zone (so it would seem).
Unless you are 1 hitting the mobs, there is no reason to even want to go higher, because they regen to full health after you hit them from outside their aggro zone.
In pvp I was setting it to 65535 (a much lower value will do the same thing), but rarely get a shot off because of the "insufficient range".
Interestingly enough, it doesn't always prevent the shot, which means there is likely hope to remove it.
I tried searching for several addresses, and codes in the disassembler that may dictate this, but have had no luck as of yet.
One interesting idea I had was to edit the coordinates of your target to match your own, or at least very close to yours (using script injection).
I believe they are found somewhere attached to the player pointer, but I have never found them.
Maybe it's time I start looking.
By the way, the correct addy is 021E866E