I ran voxel-wise analysis between two groups using randomise in FSL and the final statistical maps is as attached.

I aim to to repeat the analysis using surface based analysis. I followed the steps as in wiki. I am not sure what is the correct value for projfrac int he command "mri_vol2surf" that is more appropriate to show the findings that I have in the voxel-wise (more int eh white matter) on surface.