Thanks for your replies guys which confirm that I am basically correct.
Sycamore: the FM is somewhat vague. Because the position of our seats is variable it gives the arm as: as reqd. Despite saying that the seats should be in acertain position for landing and take-off.
Qmax: because the seats can be moved around so much, we really need the arm of seat plus pax. Plus we use actual pax weights.
The arm and station points can be considered to be in the same plane. My figure is just over 6 inches further forward than the company's.