1. Which BC coilover.
2. Generally preload is not how you set the height, height and preload are usually seperate.
https://www.ftspeed.com/part/f-20e-b...E#.YCbVRWhKiUk
See the lower collar on these, that is ride height adjustment lock collar. You loosen that and screw the damper body into the lower housing.
3. Why so low?