XP & CR for Monsters with magic equipment?

Suppose I have a monster. OK he’s not really a monster, he’s just Knight like the one on page 347 of the monster manual. Only this particular Knight has a Spellguard Shield as described on page 201 of the Dungeon Master’s Guide. Obviously he’s going to be more hearty an adversary, but how do I calculate the appropriate Challenge Rating and Experience Points for this Knight? And more generally, how does one calculate the CR & XP of NPCs with magic gear?

Thanks in advance for any assistance.