Hi Chuck,
As you may have noticed, the idea behind the basic recipe is to not have to worry about temperature, but to cook based on doneness.
So a few questions...<UL TYPE=SQUARE><LI>Are you using Kingsford?
<LI>Did you light 1/2 chimney full of Kingsford and spread them over the unlit coals?
<LI>Did you use the Weber water pan?
<LI>How many slabs of ribs did you cook?
[/list]Regarding "how do you get 33% on the vents", vent fully open = 100% open, vent fully closed = 0% open. So for 33% open, you just look at each vent and eyeball it...close it until only about 1/3 of the hole is open. "30% to 33% is it that critical?" -- no, it's not. You're just estimating it. But sometime when you're not cooking, you should play with the vents and get some practice...set to 75% open, 50% open, 33% open, 25% open, etc. It's not a science, don't get too worked up about it. I'm just trying to give you a sense of how open the vents should be.
Good luck!
Chris