I don't actively measure. I have 4 dishes of varying sizes (I'll guesstimate and say it's a total of 3 cups in volume, and I have 10 hens) and my girls seem to take about 2-3 hours to finish them off, so I figure that's a good amount to set out in the morning.
Cut back on the amount you put out at a time. That may mean having to put out fermented feed 3x a day instead of 1 or 2.
I solve the problem of how to manage fermented feed portions by only offering fermented whole grain feed in the morning, when they're hungry, and then offering dry pelleted...