The formula I use is as follows,It's easy to use once you've done it a couple of times.

Predicting the number of losing runs of a given length, against a given strike rate:

X = N x S^2 x (1 - S)^L,

Where:-

X = Numner of losing runs of length L
N = Number of results.
S = Strikerate (as a decimal fraction).
^ = To the power of.
L = Length of losing run you wish to investigate.

So for example, if you have 100 results and a strikerate of 30% and you want to know how many losing runs of 4 you should have, the calculation becomes:-

X = 100 x 0.3^2 x (1-0.3)^4
= 2.16

and for losing runs of 10,

X = 100 x 0.3^2 x (1-0.3)^10
= 0.25

This last calculation means that you have a 25% chance of 1 losing run of 10 in your 100 results, so if you continue with this system you would expect to have 1 losing run of 10 at 400 results.

It's interesting to note that it doesn't matter how high your strikerate is (except for 100%), if you work to enough decimal places you will always get a positive value, thus indicating that however small the probability, there is no length of losing run your results cannot theoretically contain.