yes , the req for magnetic personality if I am right is CH 9 and below. Unless they change it but thats kind stupid needs fix.
Its even post that way on wiki perk. so only point have any CH beside 1,3,5,7,9 would be for party size or skill effect.
I'm pretty sure it is:
(CH - 1) / 2, rounded down.
So for 3 you get 1 NPC. For 4 you get 1.5 followers = 1. For 5 you get 2 NPC, etc.
The Formula is right like this (CH/2) -1= npc followers (round up)
its even list in wiki that way, That's how they have it coded probably
But formula you have comes out the same result even tho not formula they use
if you want simplify it just know the chart or think of it this way
Ever +2 CH to base Ch 1 adds follower