Options->File Location->Audio
The base path is what's specified right there. The base path is the part of the path that does not change, regardless of the ripped content.
To specify the same place, regardless of which MC installation you use, then use a UNC convention, like:
\\nas\volume1\FLAC
The next line in the options, Folder rule, specifies how your files will be organized under the base path, according to their metadata.