i have read that if you measure from the boom round your elbow and back to the boom again you will be able to find out how long your harness lines should be... I was wondering if thats the best way and does that apply to a seat harness?
With a seat harness the hook is lower than on a waist harness, so you could go a bit longer or position your boom a bit lower. Btw, you can read more about harness lines length on Guy Cribb's website, he explains more than this rough rule of thumb (rule of elbow :-) ?
The right length allows you to sail with your arms completely straight and yet with weight in the harness. The length will vary with wind strength. Longer with light wind, shorter with strong wind. When your behind is hitting the waves they're too long. When your arms are taking too much strain the lines are too long. If your arms are bent they're too short. That's why I like adjustable lines. Easier to change the line length than move the boom.
Matt Prichard told me most wsurfers use way too short harness line.
Recommended 28-30" lines for most.
i find if the lines are too long i'm constantly unhooking in chop and can be frustrating
I tried 28" lines, boom around my nose, seat harness, and most of the weight was on my arms. 26 is as long as I can go and put all the weight on the harness. There just has to be an optimum length dependent on peoples anatomy.
Harness line length is an intersting topic. I used 22" for years and at times I even thought they were too long. When I started getting into speed I was told to go longer and progressively I worked to using 28" which feel comfortable now.
A couple of years ago I was swapped gear with another sailor who was using 22"'s and I almost felt glued to the sail, no room to move. I couldnt even fathom how I managed to use the 22" lines.
I'm pretty tall, however I've seen many sailors who are shorter and using longer lines, so I guess your stance compensates. I recommend anyone who is using short lines try some adjustables and to see how far you can go with the length.
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good choice
remember to check Guy's articles on stance to make the best of your long lines
I've always found larger sails ( 6.6 + now 7.2m ) hard to pull on and very uncomfortable and tiring.
Light bulb moment 1
When I was trying the 7.2m it was set up with the harness lines further back than I usually use ( c 5" further back! ). I found it worked well. The main reason mine were so far forward is that with the big sails I am often sailing in subplaning conditions out of the straps and standing up near the mast. I found I could still do that with the lines further back.
Light bulb moment 2 -
I had the boom at the bottom of the cutout ( I'm 5' 6") so it was about chest to chin height like I usually have it. Someone who knows the sails and better sailor than me pointed out that the centre of effort in the sail was higher than where I had the boom and that would make it harder to pull on etc. We raised the boom to about eye height ( middle of the cutout) and lengthened the lines and it felt heaps better.
The only problem now is that works for the sail at the bottom of its range but to get max downhaul I need to put the ext up a hole which made everything feel yuck again.
I cant remember where that made the boom have to be..? So the boom is even higher..???
Here's a video showing Alex Mussolini's stance with a seat harness. Its not easy to find a video where he is hooked in and powered.
For slalom I use adjustable lines - they are usually set at 28" using a seat harness. Can lengthen them when you bear off downwind if you like and shorten them up when you come back upwind. Boom at chin to nose height when upright.
For waves I use 24" non-adjustable lines and a waist harness. Never use anything but wave sails in the waves for obvious reasons...boom at neck to chin height when upright.
Am 5'10" tall. Whatever feels comfortable and right for you is the way to go. May take some time for this to happen though. 
