i would say that this factor would be determined by your resilience and + heal.
personally my goals are like this (while this may be different depending on the arena bracket:2/3/5 you intend to do)
1) soft cap resilience - i believe in order to minimize crit chance, the magiv number is 492.5 resilience. more is still good tho as it affects debuff duration and ticks
2) spell hit - at least if youare playing a disc priest, this thing is very important in high brackets. players already spec natural fear resists etc in their builds so, cappin this will help prevent dispels/mass dispels/and fears from resisting. the cap u need to hit is 38
2) plus healing - i have capped the above 2, but my healing is only at +1440 or so which is quite frankly, low. now, i guess if 2's is your bracket, it may be possible to lower your resil to compensate for higher healing but personally, i dont make that sacrifice.
assuming a full set of pvp gear (every pice of gear being a piece with resilience) u can assume the following
there will be 2 red sockets and 5 yellow sockets and 1 meta
assuming you will be gemming the IED meta, you will need to gem 1 red slot with pure healing, 1 red slot with a purple gem, and use yellow slots for either resilience or spell hit (with one green gem to cover teh blue gem requirement of the meta).
that to me, is the only efficient way to allocate gems. there is another option of course with enchanting and that is to encahnt youir spell hit on your head and glove enchant, but personally i just went with healing on gloves and resil on head.
anyway hope that helps