For the process of normalisation, how do I create an average spectra for a hypercube containing dark reference radiance data?

Assuming, i have a hypercube BlackRef(143,800,424)

I load up a portion of the hypercube using multibandread ; and the following subset

{'Row','Range',[100 110]},{'Column','Range',[100 110]})

assign it to a 11x11x424 matrix called dark

How do I create a matrix 'darkref' for reflectance calculation from this cube, which will contain the average spectra, across a line perpendicular to the scan direction? 'darkref' needs to contain radiance data from i=1 to all and j=1 to all .

Thank You