WJJ, how many feet of sheet?
I´m also interested to know.
Now for some theory, considering that the mainsheet has to be (say) 15 ft long with the boom totally opened:
A 12:1 will need 12 x 15 = 180 ft of line
A 8:1/3:1 will need 8 x 15 + 3 x 5 = 135 ft of line (45 ft less to dump)
But this is theory only. It is safer to take the advice from an experienced F31R owner.
All the best,