The crop is your second meter. DMR states that high carbon dioxide at pinning can seal the casing surface or give onion shaped mushrooms with a bulbous base and a small cap. Between pinhead and button stage, high carbon dioxide gives long stalks with small open caps.
Once pins have become buttons the cropping band is 1000 to 1500 ppm. If your pins look like onions or your stalks run long, you are venting too little, whatever the meter says.