102.4% (also known as "unhittable" or "block-capped") is the combined total of 5% natural chance to miss that all bosses have, Miss from Defense Rating (usually around 4.6% after Diminishing Returns at 540 Defense), Dodge, Parry, Block and Block from Holy Shield. The easiest way to see how close you are is to get an addon like Tankadin2 or TankTotals - these will show you the percentage "hittable" you are (amongst other things). Chances are you're a lot closer than you think!
Hey, I actually did have a question for you. I see a lot of people talk about the 102.5% avoidance...but my dodge, block, and parry only add up to like 53.15%...so am I missing something or do I just have a ridiculously long way to go?