I've set up several 10:1 mainsheets for the N20. Use 51 ft overall, and an 11 ft taper. If you are going to use a taperable line, avoid a vectran core. Vectran does not have good durability when exposed to chafe or UV. Spectra (also known as Dyneema) is more suited to this application. The easiest solution is to start with a 5/16" Marlow D2 or NE Endurabraid and strip/taper/bury the cover at least 18".

My preferred solution is to start with a 5/16" Robline Racing Sheet (can also use Salsa, Buzz, or Conception), and brummel splice it to 3/16" spectra. It takes more time to get the splice right, but gives you a soft handling line with a durable 12-strand product in the blocks. With all the drag of a 10:1 system, this route makes the most efficient use of the blocks, especially in light to moderate air.