Sorry to say, but the one who's mistaken is you.
In PPC PoS secures the network while PoW is supplementary and helps distribute coins.
PoW blocks are granted score of 1 while for PoS score is computed as
(CBigNum(1)<<256) / (bnTarget+1)
It means that any PoW block can be overwritten by a PoS one. So PoW doesn't contribute to chain security.
You can check GetBlockTrust method in main.h.
That formula that you show has got nothing to do with security.
High Hash Rate equates to higher security... that is just plain fundamentals.
Claiming PoS secures a network is like saying that NXT with its 100% PoS is the most secure coin ever devised.
So don't give me this B.S. that PoS secures the network. All it does is it secures the profits of the current stake holders. Kind of like a Ponzi scheme.