I know I'm late to the party.
CRs become less and less meaningful at higher levels, because different players/builds have such a different efficiency at these levels. In a party that cooperates well and have synergies, adding players has much more effect than adding monsters. It takes twice the monsters to be CR +2, but a party that goes from 4 to 6 can take on +6 CR. So an 8-player party at level 12 should find a level 16 encounter fairly routine. This assuming they are on the ball.
In my groups, creatures with CR under the average party level tend to be much less effective. My players love AC and defensive buffs, making it basically impossible for such a creature to hit. When I throw weaker enemies at them, I generally double the number for +1 CR rather than the official +2. That can lead very large battles, which is how we like it.