combining odds of improving to best hand with odds that he's bluffing

[jt1]

Say you are playing hold 'em and you flop top 2 pair. On the turn comes the third spade and a player raises you. You have no read on him and his position doesn't rule out a bluff. Lets say there is 8 big bets in the pot.

You know that you have 11-1 odds to improve to a boat, but the effective odds are only 9-1. However, he may be bluffing, but that only means that you'll have to put 2 big bets into the pot to see the showdown, which makes your effective odds at 4.5-1.

Does this mean that the odds that he is bluffing must be 6.5-1 or better for you to call? 11 minus 4.5 equals 6.5 hence my figure.