Right, that's another valid way to implement tags.
There's the <indent=x> tag, but it just uses a non-relative value like the <size=x> tag. I could add an <indentRelative=x> tag (probably with a shorter name) for that, or if you're already adding custom tags, you can modify the code to multiply by size.
Another option would be making an empty quad (with a full transparency texture), which would scale up and down with text naturally, and let you control the indent size from the width of the quad. So... that's an existing tag that would let you do this!
Does this information help?