Alert: This article is obsolete (before it was finished) because of latest patch in World of Warcraft since defence was removed in patch 3.0.1 style.
Make a macro with the text
/script DEFAULT_CHAT_FRAME:AddMessage(5.6-((GetCombatRatingBonus(CR_DEFENSE_SKILL))*.04+GetCombatRatingBonus(CR_CRIT_TAKEN_MELEE)),1,0.5,0)
If the value is negative you are defence capped