The only way one arrow can cover 15 to 35 is by leaving it full length for 15, and cut to about 28 for 35. It's really ridiculous if you think about it. Carbon arrow manufacturers could get much more close on spine variations if they wanted to work the process. It's easier for them to give you a wide variation, and make you cope with that by adding lots of tip weight, or let them long, or cut them until the work.
This is one of the reasons I still shoot aluminum, and others are coming back to it. Myriad spines are available as are adequate weights within those spine categories, so you can pick a length and weight you want and match spine pretty easily.
Carbons have a very long way to go in spine variety to suit me.