Don't make it too complicated, but the "Steel Modified" choke is most likely an "improved cylinder" or Light modified with lead (steel shot tends to pattern tighter than lead). I'd stick with an Improved cylinder for the 16 yard line and a modified back to about the 23 yard line or further, I only switched to an improved modifed when I moved back to the 25 yard line for Handicap. But an improved cylinder an modified should get you a long way.

Gordon