68 - depends on what you expect. I've free-floated many barrels, and some improved, and some did not. As far as to 'how much', the dollar bill trick is what I've used.
Just for thought: I free-floated a barrel on a Remington 721 (.30-06) and accuracy improved from 1 1/2" groups at 100 yards to 1 inch with the exact same loads. Did the same for a pre-64 Model 70 Winchester (.30-06) and it made absolutely no difference. In fact, I found that I had better accuracy with the barrel tension screw tightend than I had with the barrel free floated.
I'm not the 'Great Guru Of Rifles' by any measure. What I've found from shooting over the years is the importance of CONSISTANT barrel to stock contact, or lack of contact. Inconsistancey of action to stock, or barrel to stock, contact destroys accuracy.