When developing a repair in good planform alignment, you would align your repair with the aircraft's:

In the context of aircraft structural maintenance, achieving good planform alignment is crucial for both the aerodynamics and structural integrity of the aircraft. When aligning a repair, the most appropriate reference is the butt line. The butt line is a reference line used in the design and construction of an aircraft, running parallel to the aircraft’s lateral axis.

Aligning repairs with the butt line ensures that components are correctly positioned in relation to the aircraft’s overall structure, promoting uniform stress distribution and preventing misalignment that could lead to performance issues or structural failure. Since the butt line serves as a critical point of reference in the fuselage layout, it is essential for ensuring that any repairs made won't adversely affect the aircraft's performance or aerodynamic characteristics.

While the wing span, centerline, and vertical stabilizer are all important reference points in aircraft design, they serve different purposes and may not always provide the optimal alignment reference for repairs in the same way that the butt line does.