Would it be worth having a HM charm in the main charm slot or just using the Heal charm in the Main slot?

And if having a HM charm in the main slot is better, what % of HM would make a difference?

i think it needs to be 40% or higher. I am not 100% sure on that exact number.

assuming you have no HM on any other gear, your main slot charm would need 34% HM to make your acc heal for the same amount
however, since mana cost is reduced by 25% (same as healpower) then in reality, even 1HM will increase your heal efficiency, since mana is usually the limiting factor for a cata group
so imo always use your heal charm from acc slot

http://forums.d2jsp.org/index.php?showtopic=30489317&f=272
point 4