First of all, if the material and labour Charges are separable in some scientific manner, then that % will applied in all cases (Normally those co. applied which follows costing standards)
But, the contractors normally follows the specified % given by the state tax laws for the separation of labour & material such as in rule 3 of DVAT rules specified % given for labour and other charges is 20% on total contract value.
If you follow, DVAT rules, then bifurcation will be Rs. 80000 on materials and the rest Rs. 20000 on labour and Vat @ apllicable % on goods on Rs. 80000/- and ST @ 12.36%.
However, if you follow abatements given in Service Tax, then circumstances will be changed i.e., ST charged on total contract value @ 4.8% plus Cess 3% on ST.