From Mapex on Discord

1. Nerd Rage activates at <= 20% HP, not just < 20%. The workaround for this (without affecting other calculations) is to set HP to 19%.
2. Sneak Attacks are supposed to be additive. Baseline it's a +100% additive bonus, Covert Ops/Ninja makes it +150%.
3. VATS crits are supposed to be pseudo-additive (in essence). It's adding to your normal hit BaseDamage x (1 + AdditiveCritBonuses), which means BaseDamage x (1 + PerkBonuses + ChemBonuses + SneakBonuses + PowerAttackBonuses + WeaponModBonuses) + BaseDamage x (1 + CritBonuses). However, Damage Multipliers like Tenderizer, Taking One for the team, and Follow Through multiply only the normal damage, not the bonus crit damage, so you can't use the associative property here.