Marco,
Just for comparison, here is the measured result on mine, with a MileMarker hydraulic and homebrewn bracket:
Front with as good as new HD springs (225 lbs/376 mm free length) 280 mm. This means 385 kg on each spring.
With new OME 761 driver's on both sides (200 lbs/399 mm free length). Calculated 292 mm with 385 kg. And sure enough, that's about what happened.
Rear with used standard springs (330 lbs/407 and 395 mm free length) 340 and 335 mm.
With new OME 755 (280 lbs/434 mm free length). Calculated 359 mm which in reality came out as approx 350 mm.