Ive just got a set of 28.5cms lines .I wanted longer ones to make subplaning sailing in gusty conditions with my 'big'( for me) 6.6m sail easier. I find them great for that but today when the wind picked up enough to plane I found it hard to pull against the sail / use my body.
I felt I needed to put the boom higher although it was already at eye height due to the long lines. Im 5' 6.It was gusty & choppy and I only got going in the stronger gusts.
Can you have the boom too high?
Sorry but I can't get over being jealous. Saw the wind, calculated I had time for a quick session after work 
, then remembered I had to pick up my son from sport training
Windy days are too rare to let slip by.
Sorry I can't answer your question.
The boom is too high when you are standing too upright. You can't lean back enough to balance the pull in the sail and are in danger of being pulled onto your toes in gusts.
i was happy with my 24 inch + 26 inch lines, i am 170cm.
i bought a pair of 30 inch lines.
i could not get upwind to save my mother's life.
much harder to power up harness, unless you are heavy.
i had a 2 hour session with them, will not try again.
24 inch lines i slightly prefer over 26 inch lines.
i would say use 24 inch with a medium high boom.
By alterting the position of the boom hight you alter the wind forces in the sail.By lowering the boom you force the wind more into the top end of the sail thus the wind will expell out the top and less force is needed to sheet in and that is if the sail is rigged as per manufacturer, rigging the boom higher it will give you more bottom end power.adjust boom for comfortable ride adjust harnesses on boom to suit and comfort
, then remembered I had to pick up my son from sport training Windy days are too rare to let slip by. 
