I don't imagine it would be hard to calculate but there is no point. There is nothing you can do to mitigate it, glancing hits are something every melee character has to accept as something that just happens.
He didn't ask if there's a point to it. He asked how much damage glancing blows cost his pet.
"At current Glancing blows occur at about 20% of the time versus mobs that are above your level. These attacks only do 75% of their normal damage versus a mob that is 3 levels above you. Bosses are counted as "your level+3"."
So if your pet attacks 100 times, and does 10 damage per attack...
10 x 75 = 750
7.5 x 25 = 187.50
Total = 937.50
Without glancing blows = 1000.00
937.50 / 1000.00 = 93.75% of potential damage. So the pets lose about 6.25% of their damage due to glancing blows.