schema calculation

Dear all,

Kindly suggest in the following calculaton schema for purchase order.

Base value of material - 100

BED - 10

ECS - 0.2

HECESS - 0.1

FREIGHT - 10

-

total 120.3

CST 2% on total.

The requirement is such that above calculated value of CST should be loaded in material value & posted in inventory account of material.

Thanks in advance.