Updating key figures in BI Infocube from update rules

Hi,

I have a 3.x standard content flow for an infocube (9ADEMAND in SAP SCM SPP). There are quantity related key figures in cube. I am loading data for this cube from a file and the input for these quantities could be negative. I need to make the key figures 0 if the final/target value is negative. ie., in the infocube if there is a value of 20 and if I am loading -50, the final value in cube will be -30. In such scenarios, I need to make the KF as 0 instead of -30. I checked in the update rules (start routine, routine specific to that IO) but I couldn't do it. what is the best way to achieve this?

Regards,

Anu