Depending on how hard the grass threshes I have opened up (left blanks out) on the front of the concave and put blanks on the back of the concave. The rear concave blanks need to the same height as the concave so as to make the rear of the concave smooth and not grind up any more straw than necessary.
Take off the rear deflector behind the chaffer (one with the fins on ) you don't need it in grass--not enough wind to be able to deflect the chaff. Put on some eye protection and walk behind the combine while it is running. You will be able to see where the chaffer is being overloaded and then put in a distribution auger troughs to change the way the material is delivered to the chaffer to get an even spread. It's very important to make sure the right side of the chaffer isn't overloaded, that is where the majority of your good seed is.