It would depend on the bow you have and the tip weight, but you may be close to being weak on spine that way. My advice would be to just leave your bow on the lower setting. If you thought you were going to turn your bow up you should have gone with a heavier spine arrow to begin with. A stiff arrow is usually better than a weak one.
Keep in mind every bow and set up is a bit different. It may turn out that the lighter arrows spine alright for you after turning the bow up. You will never know unless you try. I would suggest once you shoot well enough to test for spine to adjust the poundage to match the arrows. If 64 lbs is what works best, then leave it there and be happy with it. This is the best way to do it. That is the real reason bows have adjustable limbs on them, so you can tune your spine better. At 28 inches with a modern bow it will not matter much for most hunting 60 or 70 lbs, both will kill deer just fine and the lighter draw will be easier to shoot better with. Especially after sitting in a stand for a few hours

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Good luck,
Paul