If I need something super tight, I will score the piece a razor knife, and cut right to the edge of the score. There are a bunch of different ways to measure or fit a joint. If I am framing, I use a flat pencil, and I sharpen it to resemble a marking knife blade or a chisel. You have a wide flat lead that way. If you keep flipping the pencil, it will wear off on the straight edge, and it will be self sharpening for a long time. You should be able to lay out a full set of stair stringers without having to resharpen a pencil. I frame with a 40 tooth Diablo blade, and they cut straight and true. I want a fine line, and I want to cut fine to the line. They cut framing lumber very well, and last a long time.
A few years ago, I was making cabinet carcasses out in the field. A carpenter ruined my saw guide to try to screw me. I used a straight edge and a razor knife, and cut right to the scored line. I did it just to show him I could. You can easily see the remains of the scored line at the edge of the cut, so you know how close you are. I use a base model 25 foot Stanley tape measure, and I always have a spare brand new one in my bag. You should know if your saw is true, and if the shoe is parallel to the blade. You should know if it is dead square. I will pin or hold up the guard when making fine cuts. It is hard to get a saw to start straight when fighting the guard. YOU should never do this, it is wrong, and it can hurt you. After the cut you can plane it, or you can hit it with some 100 grit on a block if you need to, or even a small belt sander with a fine belt. I still have pretty good vision. If you have the right kind of blade, you can drag the heel of the blade into the back of the cut, and it will steady the front for an accurate cut right on the money. I ONLY use Diablo blades. I only use Makita circular saws. Other saws are just as good, if not better. You aren't going to find a much better blade than a Diablo 40 or 60 tooth circular saw blade. A trash blade in your miter saw, circular saw or table saw is false economy. A good saw is also a worthwhile investment. A worker is only as good as his tools, particularly when one is learning.
Direct measurement, where you put a piece in place and mark it is one of the most accurate ways to measure. The finer your line, the more accurate it will be. The guy who taught me to measure and mark and use hand tools told me one thing I never forgot. He said, "If you learn to be accurate, you may eventually become fast and accurate. But if you just concentrate on being fast, you may become really fast, but you will never be accurate."
If you are careful and deliberate about how you do things, you will figure out what level of work you are comfortable with, and what works for you. If something seems difficult, or isn't working, stop and think. Most carpentry can be broken down into a series of simple steps. The guys who are saying cut it a little big and sneak up to it are on the right track. You can always make it a little smaller, but you can never make it a little bigger once it is too short. And there are different ways that you can make it shorter. The trick to accuracy is to practice making things consistently less long until you no longer have to fit them, or to have ways that you can precisely trim them a little shorter.
