Chris is right; to avoid overshoots, I set my damper to half-open (I have the 10 CFM fan), and use the stainless steel conduit plugs - they fit perfectly and I found 'em at Lowe's (you may wish to search the forum for this discussion) rather than the heat-resistant tape to seal up the bottom vent holes. Finally, just barely crack open the top top vent, about the thickness of a penny. Do this, and you never should have a problem in that regard.
As R L and Chris suggest, you do have to learn to use the DX2's damper to adapt to different cook temps, loads, and conditions. It's important to consider the "passive" draft that's flowing into the cooker through the fan when the fan's not running; regulating the passive draft is where the damper really becomes useful. In summer with a light load of just a couple racks of baby backs, I'll typically run with the damper only 1/4 to 1/3 open with the 10 cfm blower; if I open it all the way the passive draft will take the temp up to 300 without the fan ever running. Yesterday, at 45 degrees with a 20 pound turkey aboard, I ran wide open with the 25 cfm fan to keep the WSM 22.5 at 325 degrees. If you learn how to use the damper and build your fire right, the DX2 will take care of the rest.
I try to set the damper so as to achieve a fan cycle of 30-40%. That way the DX2 can adapt to the natural fluctuations of the fire. If I'm using the ramp feature, which I don't do that often, I shoot for a 40-50% fan cycle so the DX2 can throttle down and let the cooker temp drop.
Since you're in a colder climate, as I am, you may want to buy or build some form of insulation or shelter for your WSM. I've got a BBQ Guru smoker jacket for my 18.5, and it definitely makes it easier to hit and hold the temperature you're shooting for. I use a lot less coal and smoke wood for a given cook, and I prefer to cook over cooler coals rather than have the DX2 running wide-open and full blast for the entire cook.
As a bunch of threads on this forum point out, skip the water in the pan with the DX2 or any other ATC.
I never close my top vent unless I've really over-fired my WSM and need to reel it in, but YMMV.
Enjoy your DigiQ! You've entered a whole new level of cooking now.