Chain Lightning Overload chance inaccuracy
This seems to be incorrect in two ways.
First of all, the chance of chain lightning to overload per hit is 1/3rd instead of 1/5th the normal overload chance according to many sources (including simc and many high level players). Secondly, the storm totem buff should increase the base overload chance before this 1/3rd division. AMR currently adds a flat 5% overload chance to every single chain lightning hit.

I suggest changing the body of this function to the following:

= MasteryValue(ShamanElementalChance, false)
if HasBuff(StormTotem) and (spellName = LightningBolt or spellName = ChainLightning or spellName = LavaBeam)
   = Result + 0.05
if spellName = ChainLightning or spellName = LavaBeam
   = Result / 3
if HasBuff(Stormkeeper) and (spellName = LightningBolt or spellName = ChainLightning)
   = 1

Interesting… wonder when they changed that. At one point it was a 1/5 chance - must have been a while ago though and I missed the update.

I will look into it and update before we calculate anything for 8.3.