Hi Jeff,
a better way to think about it is that you want to get Bai Hui directly above Huiyin. Those are the two points at either end of the central channel – Bai Hui is at the center of the top of your head and Huiyin is at the center of your perineum.
Making that alignment clearer involves every bit of tissue between those points and much of the advice you hear around Tai Chi, BaGua etc. (suspend the head, lift the back and sink the chest, open the midriff etc.) contributes to that goal. Suspending the head and dropping the sacrum/pelvis creates a small about of traction through the torso that gradually allows everything to reorganize more or less around the central channel.
I once studied at a Yang style school that used to test the straightness of your back by putting a stick up against your spine and then getting you to flatten your spine to create maximum contact with the stick. There are problems with that method, or any method that uses external markers to gauge alignment –
1. Your body is not static, it changes all the time.
2. Forcing alignments always introduces tension – you want to grow into the correct alignment by relaxing and releasing.
Bodywork can help (rolfing, kmi). Many people have contracted tissues that pull their necks down towards their abdomen. If you tuck your chin and lift the back of your head far enough you might feel the tissue pulling in your solar plexus or abdomen. That’s an indication of something that needs to be released. It amazing how interconnected everything is in out bodies.
Trevor