I own 2 pendulums and have found them to be 60% accurate at best, same goes with other pendulum readings I ordered from others. The way you phrase the question can certainly affect the accuracy (it has to be clear, precise and not open-ended) e.g Are my cholesterol levels lower that X range? or is Y person a reliable person to do business with at this point of time?
However, if you ask future questions, pendulum sukc$$ big time and it's 50-50 most of the time. A pendulum can only pick up current energies and answers more accurately only if your mind is clear and you ask precise questions regarding the present and past. But still, there is no guarantee that you'll always get accurate answers even if you cleanse it and practice with it regularly.
I've had though two situations/questions where I asked it numerous times in 2 months and I got the same answer every time and other pendulum readers also got the same--this is when I know that this situation is set in stone and I trust what the pendulum says.
For all other questions, take answers with a grain of salt, especially if you get conflicting answers every time.