If you read the source you quote, there is an example of how the crit chance boost works:
Leather effects, on the other hand, are adjusted to fit into the new critical formulas and add +10% to maximum critical chance limits, increasing the 50% threshold to 60%. Critical chance here corresponds to the actual ratio applied when the character attacks.
If you have a 30% chance of critical, for example, that percentage is increased by 20% with Legend-grade armor, becoming 36%.
This means your crit chance is boosted by up to 20% of its current value. If you only have 5% crit chance against Velcoffer, Velcoffer Leather set will boost this by 20% of 5% = by 1%, increasing your total crit chance to 6%.
This is why I said itâs pretty much useless on low crit rates, 120% of a low crit chance will still be a low chance.