DMR's rule is to spray at the rate at which the mushrooms were harvested: for every kilogram picked, add one litre of water.
Give it in light passes rather than one drench. A heavy single watering runs through the casing, reaches the compost, and pushes out the air the mycelium needs. Time the water after picking, not onto swelling pins, because young pins that are watered directly stop, discolour and are lost.