Everything gets stored according to the order in which the object was created. So I suspect that your ‘hidden’ line was created after the node that you wish to edit using the length of this ‘hidden’ line.
If this is the case, you will need to create the Hidden line directly after you have created the curves and lines so that the length will be available to any other formulas that you create after that.
This is where the History menu item comes into play. If you open the History, you will see a long list of everything that you have created. Here is a link to a topic where I described, in detail, how to add items at a previous place in the History list: Hacking Points in Pattern Files - #2 by Grace
Let me just mention again the importance of making a backup of your work under a different name before attempting this. And also the importance of selecting the last item in the list once you are done.