prorota calculation rule for free of goods

Dear sd gurus

i am asking about prorota , it is one type of calculation rule for free of goods .

suppose generally we maintain free goods like this

if customer buys 100 qnty 10 items free( general scenario) , but prorota rule is if customer purchased some items we r giving free goods based on how much he purchased

EX : 180 he purchased so we r giving free goods like this 180*10/100= 18 ( this is ok )

so if any customer purchased 150 items just u think in this case we r giving free goods only 7 for 100 qnt

now calculation rule is 150*7/100: 10.5

so here calculated rule is just 10.5 i mean while creating sale order how free material will come ( 10 or 11)

is there any adjustments are available in system?

thank you so much ur kindly help