I've been looking at logs over at wow meter online. Lots of logs. The goal was to try to find a good number for Holy Concentration uptime, based on real data.
Deconstructor.
http://www.wowmeteronline.com/combat...7898918#buffer - 28%
http://www.wowmeteronline.com/combat...0637203#buffer - 41%
http://www.wowmeteronline.com/combat...7734984#buffer - 26%
http://www.wowmeteronline.com/combat...7995993#buffer - 34%
http://www.wowmeteronline.com/combat...1364276#buffer - 18%
http://www.wowmeteronline.com/combat...5636699#buffer - 45%
http://www.wowmeteronline.com/combat...5636709#buffer - 40%
This is a well-rounded fight with a good mix of every spell being cast for holypriests. I personally think this is the fight in Ulduar best suited for measuring uptime, so I picked the most numbers from this one. Higher numbers are backed up by higher use of single target heals. Lower numbers are backed up by more AoE heals. Remember, HC only procs of single target spells (plus binding heal).
Ignis:
http://www.wowmeteronline.com/combat...1364276#buffer - 52%
http://www.wowmeteronline.com/combat...7864493#buffer - 22%
http://www.wowmeteronline.com/combat...1334012#buffer - 21%
http://www.wowmeteronline.com/combat...7486090#buffer - 25%
Ignis is more of an AoE fight for holypriests, but with a focus on instants. The first guy in the list got pretty lucky.
Hodir:
http://www.wowmeteronline.com/combat...1364276#buffer - 34%
http://www.wowmeteronline.com/combat...1890427#buffer - 27%
http://www.wowmeteronline.com/combat...1312795#buffer - 32%
http://www.wowmeteronline.com/combat...1312791#buffer - 53%
Hodir is another AoE-heavy fight, but with room for a lot of single target heals as well. The last guy in the list spammed renew a lot, and this paid off. IN fact, renew seems to be a mana-effective aoe heal due to its ability to keep HC up.
General Vezax:
http://www.wowmeteronline.com/combat...1364276#buffer - 8%
http://www.wowmeteronline.com/combat...7141206#buffer - 23%
http://www.wowmeteronline.com/combat...7074324#buffer - 26%
http://www.wowmeteronline.com/combat...6307356#buffer - 25%
The general is maybe a silly fight to measure uptime on, considering that you don't cast many spells, and most of them being single target. Also, it was hard finding holypriests on this fight, most priests I discovered were seemingly dualspecced to disco. Ah well, was worth a try
Kologarn:
http://www.wowmeteronline.com/combat...7833370#buffer - 19%
http://www.wowmeteronline.com/combat...3658499#buffer - 30%
http://www.wowmeteronline.com/combat...7682636#buffer - 54%
http://www.wowmeteronline.com/combat...7682642#buffer - 24%
Kologarn has a lot of heavy AoE. Three out of four queried priests countered this with heavy use of Prayer of Healing. The last one also used Renew a lot. You can see who that is by the HC uptime numbers.
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I'd like to see a statistics for Patchwerk (single target heavy fight), but it seems few are interested in making stats for that fight anymore. Haven't seen any entries since april. Ah well.
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Out of all the fights so far, it seems that HC has a potential uptime for about 50% if you try hard (spamming single target heals), and around 20-40% if you don't. It really depends on playstyle though.