Holy paladins have a passive ability called Holy Insight. It allows 50% of your mana regen from spirit to continue while in combat. We also have a glyph, Illumination, which grats 1% mana when your holy shock crits but you lose 10% return from Holy Insight. I have been trying to figure out whether I should glyph this or not, so (with a lot of help from my bf) this is what we have come up with:
This is going off my un-buffed stats
Having 3870 combat regen, during a 5 minute fight holy insight will tick 60 times giving total of 232,200 regen
(5 mins x 60 seconds = 300 seconds / 5 seconds = 60 ticks * 3870 = 232,200)
Glyphing illumination will reduce combat regen by 387 per 5 giving a total of 208,980 losing 23,220
(5x60 = 300 / 5 = 60 * 3483 = 208,980)
Holy shock will give a return of 1.2k every time it crits (even with illumination glyphed, holy shocks cost is lower then my combat regen per 5). with a 15% crit chance it will crit 7.5 times (round up to 8) giving approx. 9600 return if you have a 15% crit chance (a little RNG based though)
(5x60 = 300 / 6 = 50 * .15 = 8 x 1200 = 9600)
During a 5 minute fight
Without illumination glyph: 232,200 return
With illumination glyph: Approx 218, 580 return
During a 10 minute fight
Without illumination glyph: 464,400 return
With illumination glyph: Approx 437,160 return
Even if your Holy Shock crit 30% of the time (definitely), the return is still lower then (my) combat regen.
I did the same equation to a friend who has much lower ilvl & combat regen and it still wasn't effective and again with a higher geared holy paladin on my server with the same results.
TL;DR - Don't glyph Illumination, it's probably not worth it.
Feedback welcome :)