This is what I do...
Fill the charcoal ring and get half a chimney full of fully lit charcoal and dump on top. It takes about 30-45 min to get my temp 225-250.
I have been doing several 2-1/2 to 3 hour cooks lately and I am finding I don't need anywhere near half full for my 18-1.5" WSM and it's still 225 for a long time after. I made the mistake of putting a full chimney of coals today for my cook and I had to shut it down to drop back to 225. The bottom of the chimney was red but the top coals were not even showing any white. Next time I will be pouring over unlit coals.