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