I want to acquire Tip The Scales but unsure if the ability only applies to melee. Would it also apply to lightning attacks and/or spells?
The ADCtH on Tip the Scales is attached to and applied specifically by the Vitality damage it deals. It is not global/applied by your other skills.
Think of it like the ADCtH on skills like Sigil of Consumption or Siphon Souls.
As Evil Baka said.
Another good healing devotion is the Bat. Its proc deals pierce and vitality damage AND weapon damage. The % Adcth on that skill is for ALL that damage. Bind bat proc to an attack skill that does many many hits often (like Storm box, storm totem, Wind devil, …) you get a pretty good heal.
Thank you Lord Malawiglenn!