Well, the more charcoal and air, the hotter the fire. I found when I had a 22.5 kettle with the charcoal baskets, with each basket filled with 20-25 briquettes the kettle would run around 350. One basket filled with 20-25 un-lit briquettes with -6- ignited briquettes added, you will be around 250. Just add about -8- briquettes and hour and it will go all day. I would keep the lid vent all the way open and the bottom vent just cracked...your mileage may vary.
The same principle was used when I had the big quality grill. If I wanted a lower temp, I would start with a smaller fire. If I was roasting whole chickens, I would have a stack of lit coals on each side with the chickens in the middle for a 375 smokey cook.
If you get your Weber vents shut down, it should extinguish the lit coals. Now they will be past their high heat curve and are best re-used to light your next batch of unlit coals.
Hope all this helps......
Oh, and don't fret about losing heat. Briquettes burn around 900 degrees and are know for their uniform steady heat. If you start with too large a fire (especially in a Kettle), you will never get the heat down to the mid to high 200's. Start off with a smaller fire and add more briquettes and open the bottom vent up a tad more if you desire more heat..........
