I fill up my charcoal basket then take out 15 briquets and light them in the chimney. When they are hot I will pour over unlit coals. I usually wait about 10-12 minutes before I put the lid on. Once I put the lid on it usually is at about 310-320. Once I add the meat it drops into the 280-290 range.
Weber 22.5 WSM (2) UDS, Chargriller Acorn, Pit Barrel Cooker, Pitmaker Vault, Safe and Sniper on Trailer