I used to only dump in about 15 briquettes of lite and it always ran rock stead at 260-275F.
You want to start closing your vents on the way up, preferably 50-75F away from your target temperature. If your target temp is 250F, start closing the bottom vents at around 175F, then check in 10-15 minutes, and so forth.
Also since you're WSM is brand spanking new, it will run a bit hotter for the first couple of cooks. Dont worry if you're cooking in the 275-300F, meat will still be GOOOOD.
Kamado Big Joe
[COLOR=Red]Kamado Classic Joe
Kamado Joe Jr. [/COLOR]
A bunch of Webers