VAT Computation for Contractor under KVAT

Friends,

My friend is an electrical contractor executing works of " Electrical Pole erection and power transmission" which mainly includes

70% Electrical work 

10% Civil Work &

20% labour...

If suppose he has purchased Electrical Materials of Rs 6 lakh ( Vat paid -Rs 24000)  for the total works contract of Rs 10 lakh.. How is Vat computed & how to file Vat return under Karnataka Vat Act. Is the total contract amount to be shown at the first instance of amount received or at the time of completion of work.

please guide me friends..

Thanks a lot in anticipation..