Contraction length and Tension

What is a good contraction length for a string that starts out around 10ft if you want it to hold tension? What if you want it to be whippy? Also, how can I stop my strings from unwinding during play and make the total string tension more consistent throughout, especial on the ends?

I make strings for sale since 2011, though I don’t think about contraction length at all.
I ‘believe’ (because I haven’t really experiment with it) a more looser setup is the best in holding tension, though that will reduce the weight and thickness of the string a little thus less whippy. You need to find the balance. Personally I never worry about holding tension, but more on how the string grips the response and the weight of the string. For me, maintaining tension is more about the player and I never had any tension issue with any string brand.
Also with the string unwinding or tightening during play, it depends on what kind of tricks you do. Some tricks have certain elements that either tighten or loosen the string. For right handed players, the string usually tightens over time, but not always.