Agree with everyone above - I have tuned a few bows going into with a set arrow length I wanted to shoot. I was able to work it out, but only because I had a lot of arrow options to work from in my stock already.
Otherwise - as they have all said, tune the arrow to the bow and the let the bow tell you what works. If full length is what tunes, then so be it. Carbons are very sensitive to shortening...it can change spine drastically. So much so, that when I tune carbons I cut them in 1/4" increments. It's tedious, but works better for me.