The first rule must be the conscious and proactive use of plain English that the patient can understand at their level of education. Remember that even percentages confused some students at school. They may be frightened of numbers.
As soon as the recipient does not understand, the patient doctor communication and trust has been broken. It may not appear so at the time according to the adoration in their eyes but behind that, the passive aggressive will feel that language has been used as a weapon against them.