If you ask me, 8000 is a bit much unless you're a Viking or something that's bringing in the prestige constantly from raiding. By the time you start having that kind of prestige, you're already an emperor many times over. Sort of defeats the purpose, no? 8000 isn't the worst thing ever, but it could stand to be somewhat lower at say 6500. On the flip side it's a pittance to form a custom kingdom, which I somewhat understand because they were many places such as the British Isles where anyone with control over a fair amount of land called himself a king. Probably the fairest thing might be to add some religous head opinion as a factor like you have with the HRE decision for empires and kingdoms. It may also not be a bad idea to have some decent vassal opinion as another requirement versus prestige, which would represent your vassals seeing you as a legitimate ruler.