One of the best measures/tests is 'how do you feel"? If you feel a shortness of breath or have problems with dizziness or head aches the 50 (resting) HR may be too low, if not, you have a very good cardiovascular system. Like blood pressure, lower is better if it doesn't cause any of the above or similar symptoms.
If you have any troubling symptoms, such as the list above, you should discuss with a doctor. A resting HR much below 50 should be checked by a doctor even in the absence of troubling symptoms, I believe.
Oh okay. Thank you very much!