Create own shipping module or shipping rate calculation?

[newuser]

Country	FIRST 500GM	NEXT 500GM	Max Weight
ARGENTINA	67.00	23.00	20
AUSTRALIA	43.00	14.00	Not Limit

I have a shipping rate calculation based on the table content shown in above,
initial fee will be charged for first 500gm, and  extra charge will be added in total charge with each 500gm more.

So the mathematic logic is:

For instance(according to the mentioned table content)
Country = Australia
Weight = 1450gm or 1.4kg
So the total charge will be 43+14+14= $71.
Notice: the max weight is unlimited (looping logic required?).

My question is how do I create the shipping module based on the mentioned logic?

I am a newbie of using virtuemart, sorry for my poor english, thanks:)