As for Glancing Blows, vs. a raid boss your chance of glancing blows are 40%. It's fixed at this value. A glancing blow cannot crit, and a glancing blow can only be an auto attack. If the difference between your weapon skill and the target's defense is 15, a glancing blow deals 30% reduced damage. For each +1 weapon skill you gain (reducing the difference by 1) your glancing blow damage reduction is reduced by 3%. This means that when the difference between your weapon skill and the boss' defense skill is 5 or less, your glancing blows still exist, still cannot crit, but the damage reduction is 0%, AKA they do normal auto attack damage.
The formula for glancing blows is:
=((x-5)*3) where x = the difference between your weapon skill and the target's defense skill.