Hmm… lol, let me rephrase.
Adcth is counted from the whole damage output of a skill, not just from the weapon damage portion. But the percentage of the weapon damage portion affects the percentage of global adtch applied, scaling up to 100%. So:
If you got global 20% adcth (as reads in the second page of char sheet - with an offhand it doesn’t matter which hand, they both add to global):
10% wpn damage on a skill = 2% adcth to that skill
50% wpn damage on a skill = 10% adcth to that skill
100% wpn damage on a skill = 20% adcth to that skill
100000000% wpn damage on a skil = 20% adcth to that skill