That depends on how much stretch is in the string. If I am usining poly I usually go with 9 rackets lengths for the mains (20+') and 8 racket lengths for the crosses (18'.) better to throw away a fort or two than 17'.
When you're dead you don't know you're dead. It's the same way when you're stupid.
Balance - 31.0 cm, Weight - 358 g, SW - 346 Kgcm^2