(ber´ko) a method for determining the percentage of total body surface affected by a burn . The formula is derived from the rule of nines ; that is, certain body areas account for 9 per cent each and the total body area is given a value of 99. The remaining 1 per cent is allocated to the perineum. The age of the patient is taken into account with the , since babies have proportionately much larger heads than adults do.