When converting to 9 point support, don't place the triangles over the old 3 point support: the dimensions are wrong.
Here are the dimensions:
Enter the mirror's diameter: 17.5
Mirror Diameter of : 17.5 For 3 point support: radius = 6.18718 For 9 point support: inner radius = 3.57217 outer radius = 7.14435 triangle cg = 5.31551
You can see that the difference is almost a whole inch.