In determining the effect of aggravation of visual disability, even though the visual impairment of only one eye is service connected, evaluate the vision of both eyes, before and after suffering the aggravation, and subtract the former evaluation from the latter except when the bilateral vision amounts to total disability. In the event of subsequent increase in the disability of either eye, due to intercurrent disease or injury not associated with the service, the condition of the eyes before suffering the subsequent increase will be taken as the basis of compensation subject to the provisions of § 3.383(a) of this chapter.