most players go with +30% to +40% Crushing Blow, some with less. Crushing Blow isn’t really damage, it just slices and dices an enemies Health. most players use CB & CF in builds to get enemy Health down to 25% to activate Demonic and/or Electrification Sets and Legend Execute affix.
if you want to get into the Billion Damage Club with Fire, lot’s of Blistering & +50% Bleed Damage will get you there with Billions of Bleed Damage. if you want to get there with out Bleed, then forget Blistering and Bleed damage and go with more % Fire and other ways of increasing damage. remember, if your Meteor is doing 400 million damage and you have Meteor 40, the three Meteors will do a total of 1.2 Billion damage, and the Meteor Craters will do another 2.4 Billion damage to any enemy in them (this is why Magnify is important, bigger AoE) and why Aftermath is good with Meteor by giving it a Taunt ability.
here’s a post from back when I got my first billion damage without CB & CF. I think I was using Ice at the time with +200% Frostbiting.