What prescription is considered partially blind?
Almost no association at all. "Blind" is an emotionally loaded word that has no place in Ophthalmology (except perhaps to scare a patient who is non-compliant with medical treatment). The degree of focusing problem a person has, can and should never be called "legally blind" or such, especially if glasses can correct that blurred vision to anywhere near normal.
What I imagine you mean is something like "How nearsighted can somebody be before they can no longer function out in the world without their glasses?" It does not take a very high Rx at all for this to be the case. Anybody with nearsightedness worse than about -1.50 diopters will be that way!
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic