Spirit Barrage Damage is tricky because you get two types of damage.
Damage 1: Rune Damage (usually Manitou)
Damage 2: Phantasm Damage (Gazing Demise)
Damage depends on FPA (frames per attack)
Damage 1:
So the base damage of SB barrage is 600%.
At 1.41 ias - FPA is 34 frames
At 1.84 ias - FPA is 26 frames
30 frames is 0.5 seconds.
So in 4 seconds of channeling (240 frames)
1.41 ias - damage at frames: 34,68,102,136,170,204,238 (7 attacks)
1.84 ias - damage at frames: 26,52,78,104,130,156,182,208,234 (9 attacks)
Damage 2:
Phantasm Damage is calculated differently - You get the phantasm rune with gazing demise
Manitou damage is 300%/per second for 20 seconds.
Phantasm damage is 750% x mask of jeram x enforcer
Phantasms accumulate damage and then releases that damage all at once
It accumulates damage every 1/2 second. It releases damage when it despawns (naturally after 10 seconds or when you cast the 4th phantasm - you can only have 3 out at any time and they last 5 seconds / 10 seconds with gazing demise)
damage accumulated at every 1/2 second (tick) - phantasm damage x Number of monsters in 10 feet radius x pet multipliers
So in a big mob, the more accumulated damage - the bigger the explosion when the phantasms despawn
So when you continuously cast you get new phantasm at these frames :
1.41 ias - 34 (phantasm 1) ,68 (phantasm 2),102 (phantasm 3) ,136 (phantasm 1) —> (4 ticks worth of accumulated damage)
1.84 ias - damage at frames: 26 (phantasm 1) ,52 (phantasm 2) ,78 (phantasm 3) ,104 (phantasm 1) —> (3 ticks worth of accumulated damage)
To maximize damage in large mobs, you need to let the phantasms accumulate as much damage as you can before despawn the 1st phantasm
Source: https://www.reddit.com/r/Diablo/comments/6egxrm/an_indepth_analysis_and_guide_on_lon_spirit/