Check out this hang calculator for details:
Plug in your numbers (distance between trees, for example) and see how high you'll have to hang your straps to get the right angle, etc. One thing to note is that this is just mathematical. In real world terms, you'll also have stretch. Even with amsteel Whoopies and low-stretch polyester straps, the further apart you hang, the more sag you'll encounter, so the higher you'll need to go above what the calculator spits out.
I've had some forum members comment to me that they feel 12 feet is the perfect distance, accounting for tarp and hammock size. 15 is almost too wide and is probably less ideal.