The orientation of the SN plane to the FH plane should be:

An alternative horizontal reference line, easily and reliably detected on cephalometric radiographs, is the line from sella turcica (S) to the junction between the nasal and frontal bones (N). In an average individual, the SN plane is oriented at 6 to 7 degrees upwards anteriorly to the Frankfort plane. Ref: Contemporary orthodontics, William Proffit, 4th edition page 207