here we go.....
the 8 valves measure at 340mm from the bottom of the metal arch to the centre of the wheel.
the 16 valves measure 330mm.
and carb models run at 360mm.
most manufacturers use the measurement drop calculated from the carb model so a 40mm drop for a 16v is actually a 20mm drop on that car, as the measurement is 40mm from 360mm.
make sense?
so to measure you're cars drop, measure the distance from the centre of your wheels to the bottom of the metal arch, compare it to either the carb model or to find out how much lower than standard compare it to the measurement of 330mm for a standard 16v.