Fortunately, HC's macro (written by Theck of Maintankadin, apparently) is foolproof. Just plug this macro in, and hit it every time you're choosing gear:
/script DEFAULT_CHAT_FRAME:AddMessage("If ratio above 1.88 start getting Parry. Currently at: "..string.format("%.2f", (GetDodgeChance()-10)/(GetParryChance()-10)))
If your ratio of Dodge to Parry rating is above 1.88, you can start picking up Parry gear to balance them back out, or if not, keep working on your Dodge rating. Like I said, foolproof.
Tanks who know what they're doing might have other insights (and feel free to share those in the comments below), but as an easy and simple guide to what you can start stacking when, I like the way this macro works. I'll definitely have it on the bar as my pally moves towards endgame.