38 C.F.R. § 4.22
In cases involving aggravation by active service, the rating will reflect only the degree of disability over and above the degree existing at the time of entrance into the active service, whether the particular condition was noted at the time of entrance into the active service, or it is determined upon the evidence of record to have existed at that time. It is necessary therefore, in all cases of this character to deduct from the present degree of disability the degree, if ascertainable, of the disability existing at the time of entrance into active service, in terms of the rating schedule, except that if the disability is total (100 percent) no deduction will be made. The resulting difference will be recorded on the rating sheet. If the degree of disability at the time of entrance into the service is not ascertainable in terms of the schedule, no deduction will be made.
38 C.F.R. §4.22