This is the way the arrow spine business has worked for me for at least the last 40 years ...
For freestyle (hard sights), recurve/compound, fingers/release aid, shooting off a flip rest, w/plunger, and a vertically held bow, I seek alignment from head to eye to string to sight recticule. This equates to a critical arrow and spine, and it may take lots of testng to find the right combination for me (or for anyone).
For barebow off the shelf, or hand, and a canted bow, my aiming criteria is different and I seek to align my dominant aiming eye over the shaft, aiming the arrow via some mystical "trajectory/instinctive/voodoo" aiming process
. This does not absolutely require a critically spined arrow. Period. I can make 290 grain 1716's with mylar vanes shoot like bottle rockets out of the 43# Saturn, with dart-like stable flight (though I must have my act together for form!). Likewise, I can shoot 550 grain 70/75# spined 11/32" woodies fly to short marks consistently, albeit slowly when holding 43#. Again, the reason is I'm aiming the shaft and can compensate with bow cant to get the arrow to shoot where I point it. Form definitely still counts! YMMV.