Dear Bastiaan,
The "target line" IS the aim of the putter face (wherever the putter face is in fact aimed), and the body (head to toes) is squared not to a "target line" but to the aim of the putter face. If you are using the proAim glasses, then 1) aim the putter face, b) set your head so that the parallel lines of the glasses match the aim of the putter face and are centered on either side of the putter's sweetspot, then c) turn the head so that the two lines do not "skew" sideways but both run straight [the grass blades apparently covered by or run over by each of the glasses' two line segments during the head turn all form a straight line] -- this will show you where you have actually aimed the putter face.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
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