Dowsing charts are diagrams that are used with a pendulum where instead of Yes/No, you write many possible answers to your question. You can find some examples of you search "dowsing charts for pendulum" on Google.
I wanted to know if I should use them to get a more accurate answer to my question or do they reduce the accuracy of the pendulum.
Yes, in my experience, they work well. I've always asked questions about things I can easily verify, for practice. For example, the position of something, or any question about something I don't know at the moment but can easily verify without divination, just to test my ability to get truthful answers from the pendulum.