Is 12 cu. in. LB PVC Conduit large enough to splice 12/2 run?
You don't need a clamp allowance because there are no cableclamps here
There's no need for a clamp allowance here, because that is for cable clamps that sit inside a box and anchor the jacket of a cable to the box's entrance, and you don't have anything like that in this scenario.  So, you have 4 2.25 in3 allowances for the 12AWG wires + a fifth for the grounds, which makes 11.25 in3, within the range of this 12 in3 marked body.