Regarding rhubarb analysis by BP Monograph

while a search for analysis method of rhubarb…i found BP monograph but I do not clearly understand several phases about the method can anyone help me to explain…how at first say that the assay “The underground parts are often divided; the stem and most of the bark with the rootlets are removed. It contains not less than 2.2 percent of hydroxyanthracene derivatives, expressed as rhein (C15H8O6, 284.2), calculated with reference to the dried drug”.
and then he says"Calculate the percentage content of rhein from the expression"
and after the equation, he makes a note "i.e. taking the specific absorbance of rhein to be 468, calculated on the basis of the specific absorbance of barbaloin.