hey chap, its realy hard to say, im involved in a thread titled
"coilover math" which started with a very simular question about preload on k-sport coilovers.
the ratio of height increase to shortening of spring is highly dependent on the strength of the spring, my coilovers have soft springs so i get about 5mm to every 2mm i tighten, but this difference increases as it gets tighter.
the way i workd it out in the end
(as my suspension is different from yours) was to jack up the car stick a piece of tape on the arch and the wheel
(as fixed marking points at 10cm apart) lower the car back down and check clearence.
the reason i put the markers is because each corner of the car weighs different amounts, so if you increase the drivers rear by 10mm then go do the passanger rear you will find the driverside, which is heavier on my N/A, is still only a fraction raised where the passanger side will raise higher.
with the markers in place you can measure the height difference on level ground as you know with no load the markers are 10cm apart, this will highlight the differing weight on each corner for you.
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.then you need to tighten accordingly!
i dont think theres a definitive measurement calculation, just ball park figures and as i mentioned its different for different spring rates and brands of coilover
