**Caveat:** this is not a complete answer, just a call for Matlab gurus to jump in here.  

Let $Y=Y_1+\dots  + Y_{11}$, where each $Y_i$ is the [uniform difference distribution](http://mathworld.wolfram.com/UniformDifferenceDistribution.html).  Note that the uniform difference distribution is a nice piecewise linear function.  We simply want $P(Y \geq 8)$.