On my initial lighting I let my temp overshoot my desired temp a little/ I am not concerned if I overshoot 225f. The KK is not heat soaked at all so the temp will drop down after I set my vents.
I let the KK(23") heat soak a good while after lighting at least an hour or more.
After putting that much meat on the temps will drop for quite awhile. The problem with the fan is that it will get to much charcoal lit to get back to your desired temp.
I have a fan controller that I don't use anymore but when I did I only turned it on after the KK was fully heat soaked. With the fan controlling temps the bottom vent was closed and the top vent just barely open so I can feel the fan pushing the air out.
Many low and slow cooks,,cook anywhere between 225f and 275f as long as your in that range, all will be fine.