Spine selection is based on a 100gn point and no additional insert weights. If shooting a heavier point or insert weights, a stiffer spine may be needed. It is acceptable to use a heavier spine than recommended, but not a lighter spine (ie. 300spine OK if suggested spine is 350). For every 25gn of point/insert weight added, increase spine by 50 (ie. draw weight 70-75lb at 28" with 125gn point should be 250 spine, not 300). Arrow length is measured from the throat of the nock to the end of the shaft. Please contact customer service or visit and authorized dealer for proper arrow selection and support,