The Secretary shall adopt and apply a schedule of ratings of reductions in earning capacity from specific injuries or combination of injuries. The ratings shall be based, as far as practicable, upon the average impairments of earning capacity resulting from such injuries in civil occupations. The schedule shall be constructed so as to provide ten grades of disability and no more, upon which payments of compensation shall be based, namely, 10 percent, 20 percent, 30 percent, 40 percent, 50 percent, 60 percent, 70 percent, 80 percent, 90 percent, and total, 100 percent. The Secretary shall from time to time readjust this schedule of ratings in accordance with experience. However, in no event shall such a readjustment in the rating schedule cause a veteran’s disability rating in effect on the effective date of the readjustment to be reduced unless an improvement in the veteran’s disability is shown to have occurred.