"I would suggest buying enough to do both front and back, then add them to the front to see if it gets you where you need to be...."

Good approach. The QFT IFR jet packs come with a set of 4 so I will be good there.

"why are there IAB's on the "secondary" metering block?
They still affect the idle circuit, so I am guessing that the IFR's on the secondary side are still affecting the idle circuit the same way as the primary IFR's are too."

Yes, I agree the function is exactly the same as on the primary side. On a mechanical secondary carb, it is feasible that the throttle is open just far enough to crack the secondaries. On a vacuum secondary carb, I am doubtful that such a state could be obtained but even there it may be possible. Even when in such a state though I don't think the secondaries would operate on the transition circuit since the airflow demand of the motor should quickly start the mains. I'm not certain about this, so the advice to keep the IFR square is reasonable. For now though, I don't want to have to keep pulling both fuel bowls.