Maybe I'm getting ahead of myself, as I do not have the servo working yet (waiting on my HM 4.2.4 boards) but thought I'd post my last 21 hours of smoking an 8 lb. pork butt on my unmodified, stock, WSM 18.5" and yes, I said 21 hours.
I ran fan only, with the above mentioned unit I recently printed. I cannot say enough about how tight and awesome everything fits with Tom's design. Great job, Tom, and thanks for posting a most wonderful idea and solution to many peoples long search for ideas and tools of how to implement many of the ideas generated here on this forum.
My 8# Pork Butt just off the WSM ....
Fuel: Truflame Hardwood Charcoal (lump, made in Indonesia, only sold in Australia), 4 KG bag + a little more
Ambient Temp: COLD! 32*F when started (it's Winter in Australia).
4:30 - Started charcoal
5:30 - Put shoulder on grill, went to work
10:47 - Oscillations that I can't explain
17:04 - Home for work and stoked coals
19:30 - Added fuel for security
21:07 - Hungry, upped temp to 230*
22:42 - Messed with C/F feature in setup (don't know why)
22:47 - Got curious so checked temp with Thermapen. Spot on.
24:00 - Thermapen again
1:45 - Upped temp to 250. Really Hungry!!
2:30 - OFF THE GRILL, WRAPPED, AND RESTING!!
What an awesome tool! I can put a Butt or Brisket on in the morning and go to work without worrying!