The insert gets glued in after the shaft is cut to length so you weren't too far off base...

The wingspan/2.5 formula is accurate to within 1/2" from my experience. It always gets me the right module for a bow but there is sometimes a little tweaking in or out to get the anchor point right on for an individual. It should be good enough to get your arrows. Now you need to draw a full length shaft and mark it about 1" in front of your riser (if you're 100% sure that you aren't going to change the draw you can mark it closer to the rest). Measure from the inside (throat) of the nock to this point and that is your arrow length. Now choose what arrow you want to shoot and knowing the length, head weight (I'd suggest a 125 grain to get good FOC on that length shaft) and draw weight of your bow you should be able to go into the manufacturer's sizing chart and determine which spine weight shaft to get.