[Freesurfer] how i can compute volume of a VOI

Dear all,   i am new Freesurfer user (Freesurfer 5.0 installed). I want to calculate the volume of a VOI that is not calculated from recon all. I ll decribe you the steps that were followed. Using tkmedit and tkmedit tools i created ROIs in many slices. Every ROI i saved it as label, then i merge all the labels using the mri_mergelabels. I am trying create a volume from this "final" label using mri_label2vol, using as label the new "final" label, as template volume the origin volume (orig.mgz) but i don't have the register.dat file. How i can create it? The above way is the appropriate way to calculate a VOI?   Thank you in advance!