Hello this is my first post of many here but I'm currently working on a project and had a question come up when trying to calculate the neutral point. Raymer's HomeBuilder's version page 114 calls out multiple undefined variables including "Sref." I'm not quite sure what Sref is considering it calls out Stail (horizontal tail I'm assuming). Outside of that I'm wondering about how accurate the simplified neutral point calculation even is. Thank you in advance!