[Mano]

As sitting bull implied, each of the N hands is probably not equally likely, so you may want to adjust by using a weighted average.

For example, suppose you know that your opponent has either AA, AK or KK. There are 6 different ways of being dealt AA, six different ways of being dealt KK, but 16 ways of being dealt AK (for a total of 28 different hands he could have). So instead of just dividing your sum by 1/n your probability would be (6/28)*P(KK) + (6/28)*P(AA) + (16/28)*P(AK).

Note: if you consider each of the ways of getting AA's, KK's, etc. as being a different hand, then this would break back down to your formula, since having any one particular hand (ranks and suits specified) would be just as likely as any other (assuming no knowledge of other cards out).