A diastolic blood pressure reading of 66 mmHg is generally considered to be within the normal range. Normal blood pressure is typically defined as being around 120/80 mmHg, where the first number (systolic) measures the pressure in your arteries when your heart beats, and the second number (diastolic) measures the pressure in your arteries when your heart rests between beats.
A diastolic reading of 66 mmHg may be seen as low-normal, especially if the systolic value is also on the lower side. However, it's essential to consider the whole blood pressure reading and individual factors such as age, overall health, and any existing medical conditions. If you have concerns about your blood pressure or health, it's best to consult a healthcare professional.