Spine is affected by draw length, draw weight, tip weight, and arrow length. It's best to figure out what approx. weight tip you'll need with your currect setup to get them near the length you want them. Then, you bareshaft test for the ideal spine. Adjustments are then made by cutting one arrow, a bit at a time, until you have the perfect spine. Then you'll know the length to cut the others.

The best way is to purchase shafts, after figuring out what weight tip you prefer to shoot, while taking the other factors into account. Otherwise, you're automatically stuck with a smaller range of tip weights that may or may not be ideal. We might be able to give you a bit more general guidance on what tip and length will be close if you give us all the variables I mentioned.