Yes it sure depends on the play style ofc. 12k looks low. I am at 15k and 46% buffed mastery but i hava 585 gear ilvl with hc trinkets ofc. There are a few cases in HC that I run out of mana that are not a problem. Higher mastery makes hots stronger and compensates for the need to heal continiously plus makes you able to deal with big instant aoe damages (ie. Thok, Siege, Garrosh).
So my experience says that 15k - 15.5k spirit is ok and 44 - 46% mastery is very good. I did a lot of test runs testing different values etc. Go for 13k, then 14k and see how it goes.