Here's something else you can do to shorten up the amount of mainsheet you need; hang your top blocks from a longer tail or leash or strap or something, off the boom. Every inch you can lower the blocks saves 9" of sheet on a 9-1 main set up.
This will cut down the amount of rope you buy but it will have no effect on the amount of sheet sitting on the tramp when sheeted on. The loose sheet is proportional to the max travel of the mainsheet blocks x 9 not the total seperation. If you set your length as per Macca's suggestion, both systems (with and without a strop) will have the same amount of loose sheet when the main is cranked on hard.
