[irchans]

JayP's formula is correct for figuring out the probability that 3 of the 5 board cards will be in your suit. (5.77%) You might also want to add in the probability that 4 or 5 of the board cards will be in your suit.

P(4 of 5) = 11/50 * 10/49 * 9/48 * 8/47 * 39/46 * 5
= c[11, 4]* 39 /c[50, 5]
= 0.61%

P(5 of 5) = 11/50 * 10/49 * 9/48 * 8/47 * 7/46
= c[11, 5]/c[50, 5]
= 0.02%

So the chance of making a flush in your suit starting with two suited cards preflop is

5.77% + 0.61% + 0.02% = 6.40% = 1 / 15.6

Cheers,
Irchans