Grandfather gives 100% multiplicative crit damage

So i'm running earthquake barbarian, ofcourse i have a earthquake aspect like

Your Earthquakes deal 35-50% increased damage and their damage is converted into Fire.

Doubled for 100%

Does it at any point even make sense to swap it out for grandfather and how do i calculate this?

Nope, you need the aspect + heavy hitter temper which outweighs GF.