Null points calculation it not depends of any other parameter, not even tonearm EL.
Whaaaa???
you need to specify at least four parameters which typically are alignment type, inner groove diameter, outer groove And Pivot to spindle (P2S). The math then returns you two null points, effective length (EL) and offset angle. The overhang (OH) is then found by subtracting P2S from EL. The knowns and unknowns can be reworked based on the information available. Effective length can be substituted with a known overhang and the P2S returned.
dave