The length of the top string generally depends on the weight of the top. Specifically, the heavier the top, the longer the string. So for a wood top you'd use a longer string than for a lighter plastic top. Otherwise, you'd have a difficult time getting the top to spin perfectly vertical right after the top leaves the string. The string that normally comes with the Duncan Imperial plastic top is about 50" which works well, especially for children. The Spintastic string is about 64 or 65" which is actually a little too long. So if you're having trouble getting a perfectly vertical Spinner or Boomerang, try shortening the string by a few inches. However, the 65" string should work well for the heavy BC wood top. I just bought a Duncan Ripcord last Thursday while in the Los Angles area and I also bought the Duncan Imperial with the new packaging and which says "Made in the USA" on the package (they used to be made in Mexico). I noticed that the string that came with the "USA Made" Duncan Imperial was about 60". That is way too long for that weight top. The old Duncan Imperial plastic tops from the 60's were considerably heavier. They came with 60" strings which were about right for that weight top. The 60" string also works pretty well for the Monarch tops which are about the same weight. But for most people 60" is too long for the newer and much lighter Duncan Imperial. Try shortening the string 50" to 55". By the way, this is all assuming the top was released vertically from your hand during the throw, i.e. with the point pointing straight up. You could also compensate for string length by releasing the top at other angles so that it will return vertically. It's more difficult but something you may want to experiment with also.
