SlicerMorph / GPA - select samples that form the mean shape?


currently I am analysing several skulls; the plan is to use a subgroup of these
skulls for generating the “mean shape” and then to compute the GP-distance of all
skulls based on this selected “mean shape”.

Is there a way to accomplish this?
There is a way to exclude landmarks in the GPA module; I haven´t found an option
to select the samples which should be used for “mean shape” computation.