You used LTP of yesterday or VWAP at 9:29 closing ? Because if used VWAP then there was no sell trigger as nifty was already trading below 4900. Or does it also means that you could factor in last 15 min(9:15 -9:30) of trade for triggers.
I have been observing the nifty according to above strategy and so far it seems to work like charm (ofcourse it can become much more potent if combined with other TA tools as well)