I've run some tests on OBT regarding the way heal scales and noticed some possible "issues".
Info: Cardinal 78, Weapon Master 45, Clan empower 1, no armor, no dyes, blessed spiritshots used accordingly.
Skill used: Greater Battle Heal 36
a) Homu +0 / No buffs ~> Matk 612 , Heal 1242
/ Dance Mystic 2 ~> Matk 733 , Heal 1275
/ Emp 3 ~> Matk 982 , Heal 1336
/ Emp + Dance Mys ~> Matk 1199 , Heal 1343
From this test it feels as if both skills increase healing by X% and Y%, but each x,y% seem to be slightly decreased when both buffs are combined?! Weird:
612 -> 733 means a ~19% increase in Matk, and 1242 -> 1275 means a ~2% increase in healing
612 -> 982 means a ~60% increase in Matk, and 1242 -> 1336 means a ~7% increase in healing
612 -> 1199 means a ~95% increase in Matk, and 1242 -> 1343 means an ~8% increase in healing
b) There's a "common knowledge" I've seen in many l2 classic posts (including in this forum) stating that grade of weapon is what actually affects healing, and not Matk. The test I did debunks this theory, while also proving that Matk does indeed affect healing, despite of weapon grade.
Homu +5 / No buffs ~> Matk 717 , Heal 1323 Valh +0 / No buffs ~> Matk 717, Heal 1323
/Dance Mystic 2 ~> Matk 862 , Heal 1363 /Dance Mystic 2 ~> Matk 862 , Heal 1363
/ Emp 3 ~> Matk 1171 , Heal 1394 / Emp 3 ~> Matk 1171 , Heal 1394
/ Emp + Dance Mys ~> Matk 1426 , Heal 1401 / Emp + Dance Mys ~> Matk 1426 , Heal 1401
I do not know if this is how things are meant to work or even if there's some hidden math on the first issue I mentioned in order to balance/nerf healing output.
In addition, I've been away from classic for quite a while, so these may be "old news" - still, I thought it'd be nice to share results in case there's some kind of mistake requiring checking.