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You can technically set a relative path, but MOVEBAK won't honor it.
Since it doesn't accept relative path, as an alternative you can have AutoCAD always run the following code every time you open a dwg:
(if(not(vl-file-directory-p (setq fld(strcat(getvar"dwgprefix")"Backup"))))(vl-mkdir fld))(setenv "AcetMovebak" fld)
Then the backups will be saved under the Backup folder relative to the current folder location of the opened drawing.
You'll also need to redefine the SAVEAS command so that this is invoked after saving the drawing to a new folder location.
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