"too low" implies that it is a problem. 70 is lower than average but would not raise an eyebrow.
50-60 is unusual and would lead me to think, why? If you feel well it is probably not a problem. But if the diastolic pressure
is on the low side, i would consider things that might cause that even if the patient is well. Leaking of the aortic valve for instance can cause a low diastolic pressure.