Map the Pricing for promotional item based on base volume

Hi

I try to map the free scheme for an item based on base volume of an item order quantity
for example   item: A
                     Base volume:  144
                     Order quantity: 12
if customer ordered  144 means get free item qty 0,like wise he/she order for 156(144+12) they get free item  qty 1
like that 168(156+12) they get free item qty 2

Like wise every time added order quantity to base volume

Thanks & Regards,

Venu