Extruding along multiple surface normals

could someone please explain why some of these surfaces are not extruding along their normals…I have tried several combinations of vectors but some extrusions are always going along the surface rather than in a normal direction.

surface normals.gh (37.4 KB)

this is how I formed the main surface …just in case it is relevant…

sine-surface.gh (53.7 KB)

surface normals_2024Jun1a.gh (40.1 KB)

Thank you Joseph…so introducing srf closest pt makes sure that the normal is on the correct plane whereas centroids may not be…?

Not quite. Area ‘C’ (Centroid) is not the same as Srf CP ‘uvP’.

surface normals_2024Jun1aa.gh (39.8 KB)