I have a WSM 18 and started yesterday with two chimneys of lit Kingsford briquettes. When the top briquettes just started to show gray, I dumped them both in with a dry pan, wood chunks, and 100% open vents.
In 30 minutes, it was about to bury the needle on the 350 dome thermometer (which tends to read about 25 degrees low), so I dialed back the vents and slowly got it down to the 300-325 area.
The cook lasted 4 hours, and it did peter out some near the end and go below 300. I stirred the coals and opened the vents. At the end, I opened the door/hatch and used long tongs to prop against it so that it was just open an inch or so.
You can also put a chimney starter under the door/hatch area and prop the upside-down door on it; it fits just right to leave a nice air opening at the bottom. I've done that for high-heat brisket cooks with good success.
If desperate for more heat, lift the lid and slide it over just a bit to let air in there as well. I did that yesterday after the cook was over, just to burn off the charcoal, and within a few minutes it roared back to life. Soon had smoke streaming from the turkey drippings that were being incinerated in the dry pan.