What are the odds of hitting 2 pair on the flop?

[DiamondDog]

Suppose you hold 85.

To flop precisely two pair you need to flop one of the other three 8's, one of the other three 5's, and one of the other forty four cards which are neither 8 nor 5.

All together there are 50 cards not yet seen.

So the probability you're looking for is (3 x 3 x 44)/(50C3)

= 396/19600
= 0.020204

which is roughly 49-1 against.

Someone please tell me if I've got that wrong.

When I say precisely two pair, I'm excluding the possibility of flopping better than two pair (trips, boat, quads).