I measure like on the foto, with needle i mean shapened point to say how much milimeter. A good shooting fletched Arrow bends like 12 mm (0 point is at 4.2 cm) when i shoot a bare Shaft with same flex It hits the left edge of the target ( shot multiple Times).
Also the other fletched arrows wich i more accuratly gave the same spine, trimmed the feathers a bit, so hitted the middle of the target twice in a row, so It seems like 12 mm flex is good with feathers and without the spine seems to weak