If the start part includes the pipe, you should be able to carefully constrain the plane to remain at the end of the pipe. If you have no start plane, or feature to tie it to, this might not be possible.
If you can think through the logic of what you are trying to achieve, you should be able to do it. But for what I see at this point in your discussion, you will need to input some kind of data to the part to make it understand your needs for the UDF.