Fit helmet correctly

Does the reducing the distance between the head mesh and the another bem mesh will solve the below issue? if yes how? if no how?

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I'm not sure what issue you are referring to.

If you are using a template anatomy, you should NOT use the option to refine automatically with head points. Using this algorithm to fit two surfaces that are not supposed to match would produce random results. If you do not have the individual anatomy but digitized head points, you should do the registration MRI-MEG manually.

After that, you could transform the template anatomy to fit the head shape of the subject:
https://neuroimage.usc.edu/brainstorm/Tutorials/TutWarping