I have just purchased a Firebee 8.5 for some snowboard kiting, after a few outings I took my lines off and mesured them. The steering lines streched about 6 inchs more than the brake lines wich would explain why the kite is always collapsing.What would be a good ratio for the brake lines (same length as the steering or a bit longer and if so how much???)Thanks for any good advise!
p.s I would like to shorten all of them to 25m from 30m How will this affect the kite???

I always set all 4 lines the same length making brakeline adjustments with a series of knots along the braking bridle of the kite. With all lines the same length, it's also easier to stretch and setup a new set of lines.

It is normal for new lines to stretch 6 inches or longer after a few initial hours of flying. Expect to make an adjustment or two until the lines are completely broken in.

>I would like to shorten all of them to 25m from 30m How will this affect the kite???

It will make the kite fly a little faster with slightly quicker control. It will also reduce your wind window causing the kite to reach the edge sooner. You'll be able to get out of the power zone faster if that's what you're after. You'll also notice a more sudden burst and release of power or less stable lifting support if you are jumping.

This is intirely a style thing. You'll have to try it out to see if short lines are better/worse for your personal style. But it is well within the norm, most kite snowboarders fly 20-30 meter lines.