is CB (crushing blow) alone damage independent?

as it always takes a percentage of a monster's current health instead of being calculated by actual damage?

Correct. Crushing blow removes 1/4 of the target's current hitpoints (1/8 for ranged). Your normal weapon dmg is still added (applied after the CB).

Certain exceptions to the 1/4 or 1/8 apply (less life taken from players/hirelings/bosses/..) but it is always a set percentage and independent of your actual damage.

Side note: the percentage also changes (drops) when you get more players in the game because CB essentially just doesn't scale.

This article explains it nicely https://diablo2.diablowiki.net/Crushing_blow