Sure Patrick,
First, standard arrow spine is based around a 28" long arrow with a 125 gr point, supported 26" apart with a 2lb weight suspended in the middle.
Arrow spine is reduced 5# for every 1" longer than 28" and increased 5# for every 1" under 28".
Also, arrow spine is reduced 5# for every 25gr point heavier than 125 and increased 5# for every 25gr less than 125.
As far as the numbers 26 and 80 in the calculation. 26 is a constant and 80 is the spine of the arrow you're looking for.
eg. This works for me with cane and boo shafts....If your draw weight @ your draw length is ..say 50# @ 29". You will need at least a 30" long arrow, which is 2" over the std 28" = +10#. therefore, you're looking for an arrow with a 60# spine with a 125gr point. If you want to shoot a 150gr point (25 over the std 125) then 65# arrow is needed. Then do the calculation for deflection. 26/65=0.4", therefore an arrow that deflects about 13/32" (0.406) is a good starting point.
Not to complicate things more....but, I have learned (from PatB) to start with arrows about 3-4" longer than my draw length, then do the math, and always tread on the lighter side spine. This will help, if you do not shoot off of a "cut-in" shelf bow and for the natural taper of cane and boo. Plus if the arrow shows a weak spine when shot, I've got room to trim off the arrow length to stiffen the shaft when bare shaft tuning.
Sorry for being so long winded, hope this helps,
DBar
