Charisma is very good.
Everyone like you when you are charismatic, even you like yourself.
If you have low CH it means everyone think that you're nobody, noone respect you, everyone laught at you. You hate yourself that you born with 1 CH and world hate you, sometimes you think to suicide. Even brahmins are more beautiful and attractive than you. Your confidence is nothing, you don't have it at all, everyone blame you whole your life include yourself. You're very unconfident and you think that your life is one big bad joke.
Middle CH means you're like others (well... if look on most 1 CH player community, you can even consider yourself supermodel with even 3 CH... but don't take it into account), nothing good, and nothing bad.
High CH means everyone like you, you're cool, they see your actions always as the only and the right way or you're simply beautiful, or you know how to affect other people, or... you simply have high number infront of CH in character sheet. Anyway it making you feel that it's your world, and you never regret that you born here.
Also scew realism, who cares about it.
In short words:
Right now critical chance is your LK + trait/perks/aim bonus/unarmed attack ability.
To prevent some critical effects victim performs rolls, depending on effect, like EN roll and LK roll.
There's also Stonewall perk, that helps to prevent two critical effects, but it again uses EN.
Suggestion:
Formula for critical roll is:
(60% + 4*Luck)/100*Aim bonus + Critical chance
change it to:
(60% + 4*AttackerLuck - 4*(TargetCharisma-5))/100*Aim bonus + Critical chance
10 LK is the must for snipers.
10 PE is the must for snipers.
10 EN is the must for... anyone to have HP and be invulnerable to knock downs and high resistant to knock outs and have alot HP.
10 IN is the must for crafting alt.
10 CH (well 9 CH..., but I think Magnetic personality should have 10 CH as requirement, so it'll be the only way to have max. limit of mercs) is the must for leader/slaver/taxi-builds.
I want to say that it's impossible to have all attributes high.
It'll make builds changed alot and ALL 7 attributes well used. Right now you have them all on 5 +5 addiitional points +1 free point from Small Frame trait. It'll make builds really very different. 1 LK - you're dead already. 1 EN - you're dead again and even high LK won't help you. 1 ST - eat buffout to operate shitty pistol. 1 PE - permanent blindness. 1 IN - unarmed/melee only (that's why high skill should give some benefit like damage or critical chance, but only with items, so high unarmed skill won't make your haymaker fatality, but powerfist will be dangerous weapon, or... make % penalty for aimed attacks the same as ranged weapon). 1 AG - it'll be hard... but still playable somehow. 1 CH - oh noes! Sha Enin won't talk to me! I can't bring a dog with me! I can't kill poor tent robber in the desert! A guy in Fortress won't increase my SG skill on 15% without mentats!
Well formula is a bit wrong... I simply wanted to show that a character with 5 CH will be crited with the same chance as now, with 1 CH he'll have critical bursts often(no, because aimed bonus in the torso is 0... that's why formula is incorrect a bit), 10 LK won't be the only way for snipers, because your target can't have 10 EN, 10 LK, 10 CH, 10/6 PE, 3/5/7 ST, 6/7/10 AG, 5/6 IN at the same time, I made CH to work like Willpower or something not only because it'll be cool, but to make all 7 SPECIAL attributes must to have.
Or you're going to implement someday wonderful replicatable mercs with high AI to multiple your actions? So 3 CH will give you almost second you, increasing your abilities alot in crafting, combat and quests, then it makes sense to be one-man-army with 1 CH, or you can sacrifice 2 or even 4 or maybe 6 points from you to be able to use permanent follower(s)'s abilities, then 1 CH will be sneak/scout-build only.