Question for the whizes.

[VivaLaViking]

When you count your outs you count single card outs. The relevent percentages can easily be calculated mentally within a fraction of a percent. But commonly other outs may also exist, multiple card outs, when two card values are needed as for a straight. Not that you would bet on runner-runner but it must increase your overall probability. Could it be calculated as a fraction of an out (eg 6 1/2 outs)? How could this be incorporated in calculating the probability of improving yor hand?