Well, I had both the 18.5 and 22.5 WSM’s for quite a few years before selling both… and I must say they were both GREAT smokers!! I pretty much had fire/heat management down pat. I will share exactly what I did, even though it is not specifically answering your question (I used lump charcoal, you are asking about Kingsford briquettes).
First off, it depends on the temp outside. If weather is warm (60 or higher), or cold (59 or below)… makes a BIG difference… as well as wind conditions.
But simply put, here is what ALWAYS worked for me.
1) I only used Royal Oak lump charcoal. It can be found at almost any Wal-Mart, and it is cheap.
2) Depending on the length of cook time needed, I would either fill the charcoal ring ¾ of the way, or full.
3) I would then light a chimney filled with lump. My rule was ½ chimney of lit lump on warm days, and ¾ lit chimney of lump on cold days.
4) Dump the lit lump on top of the unlit lump that is waiting in the charcoal ring.
5) Let it sit there for a few minutes, and put the WSM back together.
6) I would ALWAYS leave the top vent wide open. And more often than not, I had 2 of the bottom vents completely shut, and managed the temp with the one remaining bottom vent (although on really cold days, you may need to crack the other bottom vents to keep temp up)
7) I ALWAYS ran my WSM’s with NO water in the pan, and ALWAYS covered water pan with 18” aluminum foil for easy clean-up afterwards.
Following these guidelines, I never ever had a problem with maintaining a low and steady temp. And with my 22.5 WSM, I could easily get 7-12 hours out of one load of lump… depending on amount used, and weather conditions.
Hope this helps my friend!!