A 10° swing is actually quite good.
Was that with the MES Read-out, or was that with an accurate Wireless set?
If it was with an accurate therm, and you want it to be 250°, just set it at 242°.
That way it should cycle between 245° and 255°, so the average will be 250°.
If that was from your MES Read-out, that means it was taken from the sensor on the back wall, just below center. (Looks like a little toggle switch)
If you want accuracy, get a set of wireless Therms, and put a sensor "Probe" where you will put your meat. Then adjust it to get the temp you want in that area. I always favored #2 rack (Second rack down from the top).
Note: When shooting for a particular Temp, watch a couple cycles. Then note the High & the Low. The center of that cycle is to be considered the Temp your meat is in.
I hope that helps. Any questions, PM me.
Bear