The estimator described in section 2.2.1 becomes increasingly inaccurate for sub voxel width filter sizes. As the filter width decreases equation 12 becomes increasingly inaccurate until the estimation in equation 14 is no longer valid. An alternative estimate  offers a more robust result when evaluating images with small spatial correlation. Consider the following definitions
The approach outlined in 2.2.1 calculates the quantity
Forman et al derive the alternate formula
Mark Jenkinson  has provided an extensive derivation for this formula as well as pointing out another, computationally cleaner, variation on the same theme
Equations 20 and 21 remain accurate for relatively small spatial smoothnesses and it is equation 21 that you will find in the FSL code base.