You can choose "GMsca: grand mean scaling" when you build group comparison in Basic Models (SPM button), then you have the option to choose to "scaling of overall mean" to a value A.
But I am just wondering how the overall mean value calculated from each image. Does this calculated from all the 3D voxels (including the air regions) or just the brain voxels? If that is calculated from the brain region, what is the threshold to define the brain region.
Thanks in Advance,