I’m not sure if it helps (or if my understanding of the question is correct), but I needed to make a nested tree-like UI once and I used this general approach (which seems similar to your approach).
Basically, I used a template node and created indentation as follows:
Template node:
The little indentation under the folder icon is provided by a Grid Panel. That is, the content itself is placed in a container that sits inside a GridPanel at some specified location in the grid. I suppose you could use a spacer as well. I believe both of those approaches can give you an indentation width that you could control programatically.
Result:
I just thought I’d share something similar that I worked on. Hopefully I’m understanding things properly.