Potions: I almost always purchase a refillable potion on one of my early backs, 8% of 125 is 10, so I would have to use the item 4.5 times (2 initial charges and then one back to recharge, and an extra half charge/another recall) before the mastery eclipses 45 flat HP. Another way to look at it would be to add in the effective HP from my 3 starting potions, totaling 36 extra HP, plus an additional 1.5 charges of a refillable potion. So yes, after about 8 - 10 minutes the 8% bonus healing Mastery starts doing work via potions, but I need that 45 bonus HP right away, not 8 - 10 minutes from right now.
Spells: Because I do not run AP on a typical Bard rune page, I only have 5 AP, which gives my rank 1 HP pack roughly 73 Healing. 8% of 73 is 5.84, assuming we round up to 6 I would have to use ~7.5 HP packs to equal out 45 flat HP, this denies this potential HP to my ADC, who needs that sustain much more than I do (what with additional starting gold for more potions, and an easy refillable potion purchase).
Regen from Buffs: I should never have red buff on Bard, unless I smack an enemy jungler in the face and "secure" the kill.
Regen from Tank Items: Do not get amplified by the additional 8%, it affects BASE health regen only, and tank item regen is additive, not multiplicative.
45 flat Health is more valuable to me during the early stages of the game, where I am actually strong enough to slug it out with an ADC for a few auto attacks and not get chunked for half of my life. 8% extra self healing is simply not strong enough in the early game, and not relevant enough in the mid/late game.
Again, I'm speaking only about
, but I have put a great deal of thought into this choice.