real time changing of logic for keyfigures, how to do it..

I am having an ODS with four keyfigures a,b,c,and d. a,b and c are populated directly from corresponding fields in R/3 whereas d is a calculated keyfigure calculated on basis of a,b and c. The problem is that the logic for calculation changes real time. if on january it is ab-c, in feb it is ab. This doesnt mean that the logic is fixed for different months. wat i gave is just an example. we are totally unaware whats the logic next month. How can i accomplish this in sap BW. finally the end user should be able to give his logic and get the correct output in real time. if not in BW can i do it in BEX?