[Freesurfer] extracting mean thickness values across subjects from qdec clusters

Dear Freesurfer experts,

After a qdec contrast that asked where does thickness correlate with behavioural score, I got 2 significant clusters, and I would like, for each of these clusters, to have a list of extracted average thickness values (across the entire cluster), for each subject that I can then plot externally against the score values.

I tried applying answers to previous similar questions with no success; some of them suggested using mri_segstats, however I'm not very clear on how annotation files are to be used (I've only used the QDEC GUI), and I wasn't able to work out the syntax that would give me what I wanted (assuming mri_segstats is in fact the command to be used)

Thanks for any help!

Best wishes, Tudor