(1) The rules for determining disability are as follows:

(a) The determination of the percentage of disability in terms of total bodily impairment for any category is solely an administrative function and shall be done only in accordance with the tables of disability listed in WAC 296-20-680 and 296-20-690, or as otherwise provided in this chapter.

(b) When the industrial injury or occupational disease has caused further impairment to a bodily area where permanent bodily impairment existed prior to the industrial injury or occupational disease, the department shall award the percentage difference between the disability for the category of impairment which preexisted the industrial injury or occupational disease and the disability for the category of permanent impairment existing after the industrial injury or occupational disease.

(c) Neither the combined values chart provided in the guides to the evaluation of permanent impairment nor any other formula for the combination of the disabilities to different body areas or organ systems used in any other nationally recognized guide for determining bodily impairments shall be applied in computing the amount of disabilities to be awarded under these rules.

(d) Except as otherwise specifically provided, a percentage of total bodily impairment in one body area or system shall not be added to or combined with a percentage of total bodily impairment from another body area or system; the percentages for each body area or system shall be stated separately.