VDA based on minimum points or actuals

Variable Dearness allowance must be calculated based on CPI.The system already doing it. But i want to maintain mimimum 19000 as base points for certain date like 01.07.2010 to 30.06.2011. But if 19000 exceeds it must calculate the actual (what is the DA points for that particular month) or else it should calculate a minimum point of 19000 (as Base)

How to define this into the system.

Thanks in advance