You get 1% of mastery for every 177.77777… rating. Elemental Blast’s buff gives you 2,400 rating to the secondary stat that it picks, so using this logic, we can divide 2,400 by the rating required for 1% to get the mastery % increase of the Elemental Blast mastery buff:
2400 divided by 177.7777 = 13.5%
86.5% rating from gear + 13.5% rating from the Elemental Blast buff = 100% rating.
You get no benefit from having mastery above 100%, so 86.5% serves as a ‘soft cap’ so you don’t waste any of the rating from Elemental Blast’s buff.