My personal experience is standing behind the pitcher, just slightly off to one side, so you get a clear view of the pitch going to the catcher seems to be the most reliable without getting any false reading. How accurate? Not sure as that would take another radar or 2 to compare things. During a game, an angle from behind the backstop opposite side of the occupied batter's box seems reliable.
Accuracy of speed? Well, all will have some rate of variable. You simply can't help it. Consistency is key to comparing pitchers and like pitches. Those blank readings and obvious misses can be frustrating. It's all relative. If you are trying to achieve some personal goal, simply check the pitches apples to apples--------the same way each time.