340's are better if your planning on shooting 70 lbs. if planning on shooting 60 lbs you could even go higher. Adding arrow length will get you to the same place. It is possible that your mathews did shoot an over spined arrow well. Every bow is different. I would suspect thought that a slightly lower spined arrow would have shot even better out of your mathews. In my opinion, paper tuning is over rated. As said before, when paper tuning, if you move 2 or 3 feet forward of backward from your original distance you will get a different result every time until you are so far away your arrow has stabilized. however, it is good as a starting point and to determine if there are major things off with the set up (in this case spine). Bare shaft tuning will also yield a similar result for your set up at this point, as it is specifically designed to help determine the correct spine of the arrow. Your going to love that bow :-)