DirectX Tessellation Cracks

I have the following simple patch function in DX11, but I keep getting rips, and when I look at the wireframe its clear that adjacent edges are not getting the same tessellation factor. The CalcTessFactor() function just does a distance from the camera to the point passed, so should always give the same value for the same edge center that I pass in.

PatchTess patchFunction_Far(InputPatch<VertexToPixel_Far, 3> patch, uint patchID : SV_PrimitiveID)
{
PatchTess pt;

// Compute midpoint on edges, and patch center
float3 e0 = 0.5f*(patch[0].WorldPosition + patch[1].WorldPosition);
float3 e1 = 0.5f*(patch[1].WorldPosition + patch[2].WorldPosition);
float3 e2 = 0.5f*(patch[2].WorldPosition + patch[0].WorldPosition);
float3  c = (patch[0].WorldPosition + patch[1].WorldPosition + patch[2].WorldPosition) / 3.0f;

pt.EdgeTess[0] = CalcTessFactor(e0);
pt.EdgeTess[1] = CalcTessFactor(e1);
pt.EdgeTess[2] = CalcTessFactor(e2);

return pt;
}


My patches are triangles.

Is there something I'm doing trivially wrong here (like assuming that EdgeTess[0] is correctly assumed to be edge 0-1, rather than edge 2->0 for instance ? - its a wild guess..

pt.EdgeTess[2] = CalcTessFactor(e0);