Loose the foil. It is very difficult to over smoke in an offset if your burning splits and you have a clean fire. I would extend the exhaust stack down to grate level this keep the heat and the smoke lower in the pit and that is where you want it and run the damper WFO choking the exhaust will give you a dirty fire.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC