I just posted a new thread for Goldtip spines on carbons because I just upped my grains and length too.
First, figure out what spine you need for the length of the arrow you want to end up with, from the chart.
Next, find an arrow, that when the grains per inch (almost all charts list that) multiplied by the length, + about 70 grains for insert and fletching + the point weight you want, adds up to the total weight you want to end up with. Hunting arrows will weight more than target arrows so be careful with the charts!
Next, go about 1-1.5" longer than your required length and have the inserts and tips installed. Cut them down from the fletch end 1/2" at a time until they bareshaft SLIGHTLY WEAK (hitting to the right of a group) because when you add fletching, they will stiffen. If, after fletching, they are still slightly weak build the backplate out until they fly where you want!
Hope this helps!