Firstly the good thing about FS mixers, they have adjustment. Mixers without some type of adjustment quickly go out of tune. Using foils it is essential to be able to make changes at the mixer to keep it in tune and to also compensate for changes in the kite's bridles or too tune the kite alternatively. Well done Flysurfer, boohoo most late comers.
Now the problem with flysurfer mixers. The way they test for changes in the pulley lines or A and Z mains has a couple of problems. First the pulley lines are folded when tested. Any shrinking in the pulley lines is only accounted for at half what it actually is. Strange as it is and one of the quirks of the universe that. Why is this a problem? Well most all foils depower onto A and B. Let B out too much and the kite inverts and is unstable depowered. Not enough and you miss out on some depower. To limit B the pulley for B will hit some limit, usually the end of the pulley line. But then there is more advanced foils, these can fly on A alone which is most flysurfers. Now maybe for German efficiency or just to keep the slightest of tension in a part of the B bridle, flysurfer too sets the limit of the B pulley to be right at the limit of B on the kite. Both these cases have a problem. If the pulley lines shrink the limit for B becomes less and so with it the depower. Now flysurfer has adjustments but unfortunately they only account for half the change, as explained. One good thing about the FS mixer is the limit for B is actually C, as the B pulley line is a bit longer than C it's limit is never reached, only that of C is. So we only need worry about C pulley line, sort of:-) Now lets talk about the worst case, C pulley line shrinks 10% so 10cm. After a mixer test and changing for the ratios of B it only leaves it 2.5cm too short. Not much? Well it actually is a lot. It's the same as if you permanently limited the effective depower throw by 10cm on any kite. The effects are more stally, loss of low end, loss of top end, slower turning,but take note the kite will be more stable! Now into the next problem and how to fix it. Z loves to shrink, this is the main problem with kite bridles, but also Z main will shrink. Z main being shorter than A main has the wonderful effect of making the B and C mains even shorter after a mixer test, further reducing depower. Ah but now what if I just made Z longer and did the mixer test again? well that would fix it all. But how much? Well you basically need to extend Z as much as C pulley line has shrunk plus A and Z main difference. You could extend it a little more to account for some shrink on the kite, but that is very touchy feely and restretching all the bridles is a good route for that you can even restretch Z main. After you have extended Z, a mixer test will have extended everything to the right lengths. Now you may run into the problem that C( thanks flysurfer) has run out of adjustment, you may need to make a very small 2-4cm extension to be placed on the end of the C main , the end of it then becomes what you line up on a mixer test. If you were to place it lower on the C main the B pulley line would now be limited so don't do that. But wait how do you extend Z? Make a knot at the desire extension length on the rear main/pigtail and move C pulley line there. Effect is Z is extended but you will need to adjust your line lengths as this changes them. There is of course other ways to make this adjustment. You could extend C pulley line, but you will also need to extend B pulley line. You could do this with one extension for both on the front. Could be simpler. But by moving the C pulley line to a knot you can move it to other knots or the ELC or the Z main rear main join and change the kites profile but also the line tune a bit as well. Lastly you could change out the pulley lines for new ones and this will fix the problem. But they will shrink quickly too.
One more thing? Have you ever changed pulley lines done the mixer test and the kite was unstable? Well the reduced depower on shrunken pulley lines is more stable and old foils become porous and this is most noticed as loss of rigidity when depowered, i.e. things flap and collapse.
Too much? Maybe paragraphs would have helped?:-)
Have a play with this
UPDATED
EDIT HOW TO DO
So the quickest and easiest way to correct for this is after a mixertest/level to measure the difference between A and B when fully depowered. Basically stand on the frontmain or line and pull A and B up and measure the difference. Your best make the Z mod before you do this, but place the C pulley line end on the rearmain/zmain join. This is because it will shorten the C pulley line a little because of the larks head and so change the AB measurement. Anyway you measure the AB difference fully depowered and it should be 25cm ( 1/4 of the 100cm C pulley line) if the pulley lines are the correct length. They won't be so you move the Z main knot down 4x that difference, measured from the bottom of the Z main to the bottom of the knot. If you have done something like a long mixer test you can add the difference of B to your AB difference, it will probably only be 1 or 2 cm but is not as important as correcting the mixer. Then redo the mixertest and adjust your flying lines. As stated before you may need to extend C main if you run out of adjustment. Finally remeasure the AB difference to check it is correct.