starting with one of the cards ie using 4 board cards, by my calculations about 1 in every 250000 hands.

starting with 2 cards (using 3 board cards) about 1 in every 20360 hands.

these numbers could be a little off, as a matter of fact i think these are actually the numbers for a predetremined straight flush, ie a royal flush or a wheel str8 flush, an other str8 flush ie, in the middle of the deck 5-9 for example should be a bit more probable. . .i hope these numbers are close, can anyone confirm? again i could be way off. i used the pigionhole box method of calculation.