Any comment on the Guy Cribb advice - back harness line 1/3 of way back, except sails over 7m - up to 50cm forward.
I have 9m with 240 boom - is 70 to 75cm the place? Does the "sweet spot" for the lines change between planing and non-planing?
for my 7-oh boom is 205 and 1/3 is 68 and that works for that sail
as my sails get larger, the 1/3 rule holds LESS true
8-oh 230 cm boom /3 = ~76 and i run it around 70
10-oh 267 cm boom /3 = 89 and i run it around 75
so, for 240 - 70 to 75 cm sounds right, but you should be able to feel it
the sweet spot on harness line positions for me is not related to planing nor non-planing, but rather how i have rigged the sail ...
btw am sure you meant - up to FIVE cm forward 
