How does Ethereal and Air Shield stack?

Arqade Asked on June 6, 2021

The tooltip for Ethereal says it makes 75% of non-magical attacks miss. The tooltip for Air Shield(X) says it makes X percentage of non-magical ranged attacks miss. It pretty clearly stacks somehow, as air elementals with Ethereal and Air Shield(50) get hit by less than 1/4 arrows, but it’s unclear what actual protection is. What is the exact benefit my air elementals are getting?