Unfortunately, in case of open splines (as yours), such effect will not behave "correctly"- fpItem.areaIncNearest / pItem.areaIncDistance functions will not return "desired" values (the Thickness value affects such effect the way it'll consider distance to "virtual thickness" border as the closest one). Also towards spline ends it'll not behave as expected.
To reverse the falloff, the easeOut interpolation can be used instead and it'll "limit" such effect from the other side.
vector v = fpItem.areaIncNearest - fpItem.Position;
fpItem.rotation.z = easeOut(fpItem.areaIncDistance, 0, effectLimit,fpItem.rotation.z, -atan2(v.x, v.y));
Actually, for your scenario you might useful a slightly different work-flow - the UV Surface mode with UV Transfrom - Align applied. To map your surface you might find handy the Spline mapping (thought to get uniform distribution a manual tweaking might be required - like use of Straighten Selection & Relax tools).
Hope that helps.