Calculate Discount Percentage from Discount Amount and Vice Versa

Hi Experts,

I have one scenario in Marketing Documents.At the Item Matrix i have two fields, one is Discount%(System Field) and another one is DiscountAmount(UDF).

Whenever i choose discount%, it will calculate Discount Amount.I done these thing on my customization. But Customer wants these scenario as vice versa which means

Discount% <--- Calculation Happen ---> Discount Amount

If i do these scenario on my customization system will get hanged.

Please provide some needfull solution for the above.

Regards,

G.Suresh