The amount of healing by Battle Standard is modified by healing effectiveness for twice.
E.g., I invest 1 point in Banner of Restoration and 0 point in Ardent Symbol so it heals 20 health per second for 4 seconds (4 health/time. 5 times/second. 19 times for 4 second, not 20. Totally 76 health would be healed).
If I have 50% healing effectiveness, it, for 4 seconds, would heal 20 * (1 + 50%) * (1 + 50%) * 19times/5times = 171 health.
I’ve already tested the skill and I am sure it is modified for twice.
I had 86% healing effectiveness (from an 2*2 idol), 1 point in Banner of Restoration and 0 point in Ardent Symbol. I had Titan Heart as my body armor so I do not regenerate and I was a Forge Guard so no passive bonus would affect healing and with no investment in passives related to healing effectiveness.
Battle Standard healed me 263 health for 4 seconds, which is almost the same as 4 * 1.86 * 1.86 * 19times ( = 262.9296 )
Also tested with amulet with 35% healing effectiveness. 139 health healed. ( 4 * 1.35 * 1.35 * 19 = 138.51 )