the best way (with mine anyway) i found was to hook an measuring tape over the bottom edge of the wheel running through the centre of the wheel and taking a measurement at the lip of the wheelarch. however if the ground is even slightly uneven this measurment can change quite a bit. the best way really is to ensure that your struts are the same length. i.e the remaining threads on the strut are the same for the two front ones and the two rear. or as close as is possible, mine are bang on in that respect, but in my drive (it isnt level) the measurements from the bottom of the wheels to the arch are a few mm different. in this way, unless the ground is indeed dead level (and you can never rly tell that) this isan ackward way to measure. the book i got with y coilovers (KW VARIANT 1s) suggested that the best way of measurement was to measure the remaining threads on the strut rather than the car.