An old topic (2011) which explains it better than I could do.. but raising concerns about the validity of the comparison. It's funny to read that post today, talking about Radeon Cards... and have a look at the difficulty! 
This is exactly how bitcoinwatch estimates FLOPS. I am not saying it is an acurate way to do the estimate, all I am saying is this is how they do it for what it is worth and for comparison to your other methods. Many here among us question the valididty of this estimation method.
The page simply uses the following assumptions/estimates:
1 INTOP = 2 FLOP
1 hash = 6.35K INTOP
1 hash = 12.7K FLOP
So the hashrate in TeraFLOP/s is simply 12.7 times the hashrate in Gigahashes/s.
As an example: 11,558.55 Gigahashs/s * 12.7 TeraFLOP/Gigahash = 146,794 TeraFLOP/s = 146 PetaFLOP/s
The page simply uses the following assumptions/estimates:
1 INTOP = 2 FLOP
1 hash = 6.35K INTOP
1 hash = 12.7K FLOP
So the hashrate in TeraFLOP/s is simply 12.7 times the hashrate in Gigahashes/s.
As an example: 11,558.55 Gigahashs/s * 12.7 TeraFLOP/Gigahash = 146,794 TeraFLOP/s = 146 PetaFLOP/s