How exactly does Herald of Ash damage/work? And why the lack of support gems?

So trying to get the most out of herald of ash and really can only find 4 support gems that truly alter its "damage" and all the other support gems just leave me with questions mainly due to the wording of herald of ash.

Herald of ash deals damage based on Overkill damage, with the actual buff itself capping the threshold for overkill damage at 20%, Meaning 20% of that enemies hp, so 100 hp = 20 max overkill damage, So say you deal 100 damage, enemy only has 100, well of that 1000 damage you did, because the threshold is 20% of enemies hp that overkill damage is only 20, not 900, so it now takes this overkill amount which in this case is 20, and will cause an explosion to appear from that enemy that ignite all surrounding enemies to now the base ignite damage which for a level 19 herald of ash is 23%, so 20*1.23 = 24.6 dmg

This buff only does damage THROUGH IGNITE, the explosion itself doesn't deal ANY DAMAGE, so any support gem that scales elemental damage can't be slotted into this, it deals no elemental damage, just gurantee's an ignite with a base damage equal to threshold of overkill

Yet I can slot Acrimony into herald of ash, Enemies affected by non-ailment damage over time from supported skills have 50% reduced life regen

I think some support gems should be made incompatible with herald of ash like Acrimony as its just confusing, the wording, even scrolling online reading different forum posts alot of people misinterpret how this ability works and scales, alot of people think its pointless does no extra damage, its amazing for clearing as long as you do more then enough damage to those hordes, also would love more support gem compatibility with herald of ash as there are only 3-4 you can slot in that don't require spirit, all other support gems require spirit, I don't have that spirit, now im just staring at a herald of ash with 3 support gems in it and 2 open slots xD regret using a perfect jewelers

Currently using Prolonged Duration 2 for more ignite duration, Fire mastery for +1, and magnified area 1 for 35% more area, literally no other gems I can slot in that directly benefit it without costing more spirit, I think the support gems like elemental armament should be made compatible but then again I'm not dev that may end up doing too much dmg who knows, It just feels really underwhelming at this point in time

Dialla's Desire does work as well (too pricy of course, but it does) and reduces spirit.
Swift affliction III with compressed duration might be an option too; or just swift affliction II.


Open 4 slots, the greater jeweler's orb is next to free, so you can experiment. I can't think of a viable case you'd need 5 support gems.