ride height info

[rc10fan]

Hi can anyone shed a bit of light on something for me?

When reading up on the set-up sheets the pre load on the shocks is always left blank. I realize that all track conditions are different but they always mention the ride height either 19 or 20 mm etc. im running 40 in the front and 35 in the rear on my 502 using losi oils blue springs on the front cut rears but the car still slaps the chassis when jumping and i dont hit the jumps hard. am i doing something wrong with the shock set-up. p.s im building them the standard way.
thanks