Pintail:
To answer your question. "Spine" is a standard, its like any unit of measure.
its measured by placing a shaft (if wooden) the edge grain points up, on two supports exactly 26" apart. Then a 2# weight is placed on the shaft 13" from either support and the amount of bend in the shaft is measured in thousands of an inch. This measurement can then be converted to pounds of bow weight by dividing that figure into 26 that sum will equal the bow weight for that shaft.
Here's the hard part. For that sum to be correct the arrow "MUST" be 28" long, any longer or shorter and the spine will be different. The rule of thumb is, for every inch longer than 28" the spine will weaken 5# and for every inch shorter than 28" it will stiffen 5#.
For this standard to work it must have a standard bow in mind. That bow the shelf will be cut 1/8" less than center and will be drawn to 28". So if your shaft spined .520 and you divide it into 26" it will equal 50# when cut to 28" and shot from the above described bow, that weighs 50#@28"
Is that clear as MUD? hope you understand it not difficult.
Bob