Hi @ranjan_r
Nopat is a little different from pat. It represents the net profit available for appropriation - both for shareholders and lenders. You take operating profit minus depreciation and reduce taxes from that amt by the prevailing rate which is 35%.
If you have the half yearly balance sheet and income statement then change in nopat divided by change in capital employed will give you the return on incremental capital - generally you do this for 3-5 yr periods and not shorter periods because it takes time for capital to produce results.
As far as d-mart goes , it appears that it is able to deploy large sums of additional capital at exciting returns. Valuation will always remain a topic of debate with intelligent arguments from both parties.
However if one thinks that it can continue this trend for a long enough period then there are not many businesses out there who have this ability and valuations should be assigned in line with this view.