Which mandibular plane is considered while calculating FMA (Frankfort mandibular plane angle) in cephalometry?

The mandibular plane used for FMA measurement is : A plane tangent to the lower border of mandible which connects with the menton anteriorly and posteriorly it bisects the distance between the right and left lower borders of the mandible in the region of the gonial angle. See Figure. 

The FMA angle is defined as the angle formed by the following two reference planes:

i. FH plane (Frankfort horizontal plane—A line between the most superior point of the external auditory meatus and inferior border of the orbit).

ii. Mandibular plane (A plane tangent to the lower border of mandible which connects with the menton anteriorly and posteriorly it bisects the distance between the right and left lower borders of the mandible in the region of the gonial angle). 

In most cephalometric analyses, the occlusal and mandibular planes are measured relative to the sella-nasion (SN) line, the basion-nasion (BaN) line, or the Frankfort horizontal plane. Ideally, according to Tweed, the incisor mandibular plane angle (IMPA) should be 90 degrees, the Frankfort mandibular angle (FMA) 25 degrees, and thus the Frankfort mandibular incisor angle (FMIA) 65 degrees. (The sum of three angles of a triangle equals 180 degrees.) Moreover, the IMPA angle also relates to creating additional space in the mandibular arch in that for each 3 degrees advancement of the lower incisor, 2.5 mm of space is gained in the mandibular dental arch. Conversely, reduction of the IMPA from 90 degrees, for example, to 87 degrees would decrease the available space for tooth alignment in the mandibular dental arch by 2.5 mm.