With dacron, a 64" string would be where I would have started for a 68" bow (rule of thumb is a string that is 3" shorter than the bow length, but dacron stretches more so they are typically another inch shorter to compensate for the stretch).
Having said that, if the bow calls for a 9" brace height, then that changes everything. Most "AMO" length strings will yield a brace height in the mid 7" range.
If I were to guess, I would think that a dacron string with an actual length of around 62" would get you to that brace height. But, there are so many variables involved, the biggest being the fact that not all bows are measured the same.
Put the 60" (actual length) string on the bow and let us know what brace height it has.