Fruiting starts with a deliberate change. DMR brings the room down to 800 to 1,000 parts per million, which is what pinhead formation needs, and holds 1,000 to 1,500 parts per million once pinheads become buttons.
Get it wrong and the crop tells you. High carbon dioxide at pinning gives onion shaped mushrooms with a bulbous base and small cap; later it gives long stalks with small open caps.

