Mvat purchase from manufacturer who is under sec.77(2)

My query is; our client purchase material form Videocon electronic items without vat bcoz said company under incentives scheme of vat having certificate for the same(purchase within in maharashtra) Our client sale the same electronic items to end customers/dealers without charging Vat. My Question is how to calculate vat on the basis of gross profit margin? Thax ganesh.ghadage @ aol.com