Mastery is great even past 50%. What determines the value of mastery is the general hp of your raid. If your raid/raid member sits at 50% (approx) or higher in hp, then crit is more valuable. If you're healing someone or a raid at 50% (approx) hp or lower, then mastery is more important. I decided to stack more mastery for HoF because our raid is generally getting knocked down to low health and we need to get them topped off fairly quickly. However, when mana was more of an issue, I went with crit.
A good article about the relationship and value of mastery versus crit is on Life in Group 5.
http://lifeingroup5.com/?p=2965
---------- Post added 2012-11-02 at 10:53 AM ----------
I started out healing normal MV with 5000 and then ended healing it with ~ 7500 spirit unbuffed (I'm at 8300 now). The only fight where I had mana issues was on Elegon, and it was always after an orb phase after topping people off after orbs die. I'm not a rshaman that uses TC except with Will of the Emperor, where I'm primarily casting Greater Heals to keep my tank up. I heal 10 man with a holy priest. So, there's very little time to cast LBs.