The amount of charcoal controls how long you can cook. You don't say how long you want to cook or what you're cooking. But let's say you're doing ribs for 4-6 hours. I'd fill the charcoal chamber half full of Kingsford. I'd light 40 briquettes and spread them over the unlit charcoal. Since you're cooking under 275*F, I would use water in the pan, hot in the winter, cool in the summer. I'd start with the top vent 100% open and the three bottom vents 25% open.
Doing as described above, if the cooker runs at 275*F or less, I'd say you're good to go. 275*F is a perfect cooking temp. If it runs hotter than 275*F, close one of the bottom vents. If necessary, close another of the bottom vents. If necessary, start to close the top vent just a bit. If you can maintain 275*F, I wouldn't change a thing. If you end up with all three bottom vents closed, the top vent partially closed, and you can't maintain 275*F, that means you've got a lot of air getting into the cooker through places other than the vents, e.g. around the access door or between the charcoal bowl and the middle section.
The door is an obvious suspect because they are notorious for poor fit. If the top left and right corners splay out above the surface of the middle section, gently try to tweak them into a shape that conforms to the middle section. If the left or right vertical edges of the door bow out a bit, try to coax them into a flat shape. It takes some time and patience, but your effort will be rewarded with a better fitting door. See
Parts Troubleshooting for an example.
This is the point in the presentation where someone says, "just replace your door with a Cajun Bandit stainless steel door + latch for $63." That's a fine product, but it's not a perfect product. Some folks are very happy with it, but some folks find it doesn't fit right, either. Or it cracks the porcelain around the bottom of the door opening when in the closed position. And while stainless looks good, the material doesn't affect cooker temp in any significant way. So I would encourage you to try to tweak your door into good shape first, and consider a replacement door only as a last resort.
This is also the part where someone says, "buy gasket material and seal around the door and lid." To me, this is really a last resort for the backyard cook. I'd suggest you try the stuff we've talked about above before considering gaskets.
This is probably enough to get you started, or at least cause you to think of more questions to ask. I hesitate to dive into a discussion about air leaks between the bowl and middle section, out of round, etc. All of that is covered in the Parts Troubleshooting page I linked above.
