It has been a couple years since I used my Heatermeter with my 22 inch WSM (I have a BGE now) but I had great success with the WSM (and BGE) using stock PID. I had a fan with a gravity damper when using the WSM just like you describe.
I think, if I remember correctly, I used about half a chimney of lit coals to get things started. You should get an awesome straight line at desired temp with that setup. Enjoy!
Well last night's test run cook with sausage was a disaster. I am starting another test cook now with more fuel and no water pan to see how things go. I am using a bowl of water as a heatsink to simulate food. I am going with:
Don't skimp on the fuel, I think you are on the right track there. Set it up as if you are doing a 12 hour brisket every time. The unused coals will just be used next time. Also don't skimp on the lit coals you add, make sure it is at least half a Weber chimney. Most people need to use a full lit chimney and the door propped open just to get much above 300. I assumed you are looking to keep 225 to 250, that is always what I used the WSM for. If you don't give it some fire to work with the graph will look like the Grand Teton's. As long as your WSM and fan are fairly air tight it should work great. Good luck!
I already updated my last post and thinks look pretty good. My smoker is tight. I have gasket on the door and lid. I used a mostly full basket and lit about 15 coals using minion method. I made it to 270 in about 30 minutes and have been +/- 2.5 deg ever since. Probably could be better, but my brisket won't care and that is way closer than when I did it manually.
One thing to consider by not using an automatic damper is a runaway fire. When I used just the blower on my 18" WSM I often found the fire got too hot (usually after taking the lid off and spritzing or by taking the door off to adjust the fire) and was pulling air in via convection from the completely stationary blower. In those situations there was no way to damp down the fire and I had to manually shut down the lower vent to get things back under control. This often happened at 3:00 AM on a brisket low and slow cook and was the reason I ultimately went to a blower+damper configuration.