Paul, Everyone has slightly different results. I have always used a full chimney of lit charcoal on top of the unlit, and my temps usually go right to about 200-220 right off the bat. This weekend, I used less, maybe 1/3 to 1/2 chimney, and the tempt went shooting right past 250 and I had to shut all my vents to bring the temp down.
This is exactly the opposite of what I would have expected. Go figure.
I usually use about half a load of charcoal for a medium cook like ribs, and it's enough. For some reason, this time I did a brisket, split into two pieces, and I used about half a ring, and I was right at the point where the coals were dying out and the brisket was about 178º. I added half a chimney, but I'm not sure I needed it to finish the brisket.
Anyway, my usual pratice is for something around 1 or two hours, just dump in a chimney of lit charcoal. For 4 - 7 hours, about half a load of unloit with the minion method. For a long cook, like a brisket or pork butt, load it up all the way.
I am perplexed that my temp went so much higher with less lit charcoal.