[Chimera-users] define plane question

Hi Everyone, Hopefully this isn't a stupid question. I've used the define plane command for a few residues of slightly twisted beta-strand, trying to judge the distance and twist in relation to a helix across the binding groove at different conditions. It seems to work the way I would like it to, but I would like to know in more specific terms, how the plane is actually determined. When the structure measurements data is saved, the file returns the center coordinate, normal vector, and the radius. Could anyone tell me how these values are determined? For define axes, I can follow references from ksdssp, but there's really no similar definition for the define plane command in the user documentation. Thanks, Steve