Hello there, time for calculation !
So. Let's calculate the potential power of Leslie :
We do know that there wasn't any dark users since a hundred years, and that the empire exists since thousand years.
Let's say that a generation is 25 years :
So :
(1000 - 100) / 25 = 36.
So, if every silver head child keep 1 or 2 point while he transfers the power, then she has between 36 and 72 of power (out of how much ? I assume it's out of 10, since it's the given example, maybe 20, and if someone can slap my face with the real score, I'll gladly turn my cheek)
But, it's rare ! But we saw that there are quite the number of arms/children that we sacrified !
So, two hypothesis :
One is that the case is not that rare at all, and so she as indeed 36 to 72 of power
The other one is that the sacrifices are made since more than a thousand years, so she may have even more !
As for why to makes her a sacrifice
@HarukaNazu, I believe she's the first one to awake her power, it never was the case before. Before, they had the power, but were just containers, while their big bro or big sis could control them