You must have a bare shaft to even float the shaft. This means no inserts. You must keep the nock end nocked and then install a nock on the insert end. Fill your bathtub with warm water and throw in some dish soap just so they spin more freely. You should be able to see the arrow slowly spin in the water. Sometimes you have to roll it a bit with your fingers to get it started. The stiffer side should be on the bottom. Take a sharpie and "dot" the top side of the shaft. That is the weak side. You want to shoot the stiff side down. So, your cock vane will be aligned with the dot you made.

I'm waiting for Straightarrow to reply back. I may be buying/making a spine tester. Would be interested to see if "floating" gave the same results.