This seems right. Also make sure the seed point is in the same plane normal on Z.
If that does not work can you share you dataset?
I am in a similar situation to that of Nayla.
I have a 3D data set and I follow the path:
calculate 2D vector using U_XiHat+U_YjHat
take a 2D slice normal to z
try to use the filter and he tells me I am not on a XY plane.
Can you please give me a suggestion that might point me in the right direction?
Thanks a lot.