From a paper I\'m reading right know:
... S(t+1, k) = S(t, k) + ... + C*∆ ... ∆ is a standard random variable with mean 0 and variance 1. ...
Do you have any restrictions on the distribution of \Delta ? if not you can just use a uniform distribution in [-sqrt(3), sqrt(3)]. The reason why this would work is because for an uniform distribution [a,b] the variance is 1/(12) (b-a)^2.
\Delta
[-sqrt(3), sqrt(3)]
[a,b]
1/(12) (b-a)^2