For our purposes we can calculate all the necessary information knowing and this can be approximated by
where is an estimate of the smoothness of the GRF, the number of dimensions in the GRF and is the number of in-brain voxels.
The probability of clusters in this excursion set is approximated by a Poisson
The number of voxels comprising a cluster is distributed according to
We can calculate  by calculating the product of and summed over