I know this runs counter to the idea of imprecise cooking but who wants a half serving of left over pasta? I started weighing pasta when on an exchange-based diet and found that weighing dry pasta helps you have the right amount of pasta every time.
Dried pastas differ greatly in shape and thickness but for many, a serving is listed in the nutrition panel as 56 dry grams. Just multiply this by the number of servings and you will have the right amount of pasta with no leftovers. If you find that 56 grams is too much for you, choose your own number and commit it to memory.
Local box stores carry food scales that weigh in both grams and ounces. Try to find one that has a "tare" button that allows you to cancel out the weight of the container you pour the food into.
What can be imprecise in cooking pasta? The water, the salt and the time.
How much water? Lots. How much salt? A good deal. Watch Giada De Laurentiis throw salt into the pasta water some time. How long? Fish out a piece and test it. I usually drain pasta when it is still a little under done so it can do it's last cooking in with the sauce.