In ARENA it is true, acc charm uses 100% mana and only works at 75% effectiveness.
however in catacombs it only uses 75% mana as well, so the efficiency is the same as the main slot. hence you can use an hm charm in the main, to boost the overall effectiveness of the acc heal.
I think you need 34% hm in order to make your acc charm function at the same power as your main, but it'll still only use 75% mana