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