The Weber Kettle is a very good grill. Great in some ways. Thanks for being specific about your equipment.
High = above 300, 325 is great. Best temperature for poultry. Good luck trying to maintain it for more than forty-five minutes without reloading.
Medium = 250 - 300. 250-275 is one world, and 275-300 another. No stall, or not much anyway when cooking over 275, but some intense bark formation. 250-275 is a good range for beef. I like to cook brisket right at 275.
Low = 215 - 235. The lower the better for fish. A lot of recipes specify 225 for pork, but with temp fluctuations that takes you below 215 now and then which isn't such a good thing.
So what about 235- 250? Call it what you will, it's very useful. Cooks as gently as 225, only faster. Better bark (aka Mr. Brown) too.
High = 2 - 4 hand count (450 and above)
Medium = 5 - 7 hand count (350 - 450)
Low = 8 and less (below 350)
Measuring Direct Temperatures
The simplest way is to hold your pal about an inch above the grill and count seconds: 1 Mississippi, 2 Mississippi, etc. Pull your hand at the "this is stupid," moment. Don't try to tough it out. The hand count times are approximate. You'll learn your own thresholds. By the way, it's not a competition.
Contact "button" thermometers are available from Barbeques Galore and many other barbecue supply stores. They are more "accurate," however the accuracy offered is not particularly useful to the task. Your hand measurement is just as informative. You're grilling. All you need to know is very hot, hot, kind of hot, and not hot. Don't get lost in the minutiae.Measuring Indirect Temperatures
Get yourself a digital thermometer with a probe connected to the readout by a wire lead. If you don't buy a "barbecue" thermometer, you'll need to make a stand out of a wine cork, a potato, a coat hanger, or what have you. The actual sensor is in the last 1/4" of the tip of the probe, so make sure the probe extends more than 3/4 of an inch beyond the stand.
I highly recommend the Maverick "Redi-Chek" ET-73. It's a digital, remote, dual read which allows you to read indirect chamber temperature and meat temperature as well from about 60' away from your pit. You can find them for ~$40 on the web. Don't try using the Maverick for direct cooking, it can't handle it.
A long stemmed thermometer in the top vent is not very useful. You need to know the temperature at the food's location.Building and Controlling an Indirect Fire in the Weber
When you're cooking indirect, build the biggest fire possible that won't expose your food to too much radiant heat. This is usally around, one large, full chimney's worth of coal. If you want smoke as well, put a couple of wood chunks on the charcoal grate, and cover them with some unlit coals, then cover all of it with your lit chimney's worth.
Control the heat with the vents, not the size of the fire. That means keeping the lid shut as much as possible. DO NOT FOOL AROUND WITH THE FOOD by "mopping" or "basting" or meaningless "turning." Just leave it alone and let it cook. Your thermometer will give you all the information you need.
The Weber Kettle excels at high temperature indirect cooking. It, along with its smaller, pricier predecessor the Patio Kitchen Cooker, may be the ultimate chicken charcoal machines. They do a mean tri-tip as well -- even if not exactly Santa Barbara - Santa Ynez - Lompoc - Santa Maria Valley style.
But, if you're serious about indirect cooking aka barbecue, get a smoker. It's too hard to maintain constant temperatures in a kettle for long periods. By "serious" I mean wanting to cook a few butts, and/or few briskets and/or the odd turkey every year. The Kettle is just too inconvenient for long cooks. The Weber Smokey Mountain is a great choice, but not the only one. If and when you're serious about buying I'll be happy to discuss options.Building and Controlling Direct Heat Fire
The more coals the hotter the fire. The longer the coals have been burning, the more they'll cool down. Different types of charcoals have different temperature profiles. You can learn a lot from the Naked Whiz. Check out his site. The Lump Charcoal Database -- Naked Whiz Charcoal Ceramic Cooking
This assumes you're using lump and not briquette. You are using lump, aren't you?
When you build your direct fire, leave a portion of the charcoal grate bare, so you'll have a "cool side" for holding and slower cooking.