Ordering the correct spine is one of the most important steps in a good shooting set of arrows. Below are a few good rules to follow when figuring the correct spine. Please feel free to contact me, if you have any questions regarding your spine. I would be more than happy to help you. Brian Ferguson Email: firstname.lastname@example.org or you can text or call 608-501-5913.
In most cases all you will need to do is add or subtract 5# for each inch over or under 28"s of arrow length.
If for instance you have a 40# bow at 28" and you shoot a 30" arrow you will need to add 10# of spine over your bow draw weight. This would make your optimal spine 50#. You would have the choice between 45/50 or 50/55. I have found that going with the lower of the 2 generally works best. If you shoot a 26" arrow you would subtract 10# instead of adding it.
If your bow was custom made, you will need to make sure that your draw weight is measured at 28". If your bow was made to specs at a different draw length you will need to find out what it is at 28". Normally you should be able to add or subtract #3 per inch. For example if your bow was made to the specs of 40# at 26" you would need to add 6#'s to your draw weight to make it 46# @ 28" and then use the rule above to figure your spine. If your bow was made to a 30" spec you would then subtract 6#.
These rules are based on a average tip weight for your spine. If you shoot a heavier tip than normal you may need to increase your spine to accommodate the heavier tip.