Article for calculation stock is overvalued or undervalued is nice. I would like to add few more things with it are before investing money you should track the script for a some period so that you will get the idea of its low and high and what will be the good entry point.
I will take Infy as an example currently it is trading near about 2300Rs but suppose Infy started falling down because of any reason and it come near 2100 then I will definitely buy this stock and at that time whatever may be the index it is 7800 or 8800 and from 2100 level if it will come down then I will make average.
Whatever may be the PE ratio you have to track the script before investing for certain period