I didnt exactly answer this for the Benelli choke. The Modified Benelli choke is basically a Full for Steel. I dont think you will get much tighter than this. When I use Steel shot I have the improved cylinder choke in, which is like shooting a modified with lead.
All in all I would just pattern your gun with different ammo, at different distances, with different chokes. It will take a lot of guess work out of it.