It doesn't work that way.

You're correct. The insta-kill chance would be somewhat over 60 percent, still overpowered.

Not this way, either. Instakill is 20% tops.
(but I agree that 6 luck is optimal for powergaming)

Just checked the wiki. The claim that all rounds of a critical burst get a roll on the critical effect table independently was incorrect. So it's up to 19 percent insta-kill ranged (with sniper) and up to 20 percent insta-kill chance in melee (with slayer). per attack.

Yes, you have no idea. Sniper with 10LK means every shot is critical. With Better criticals, there is 20% offset in critical damage type, which means insta-death. A typical minigun shoots 40 bullets, each of which has 20% chance for insta-kill. This is how I fallout, but you can continue spearing.

You can always use Per's Ultimate Fallout Guide to create your character.

The link is:

https://nma-fallout.com/resources/pers-the-nearly-ultimate-fallout-guide.80/