You generally want a page margin to be a fixed width. In your case, 5% of the page.
When you indent one line of poetry relative to another line, you want the amount of indentation to scale with the text:
eg:
Code:
Line 1
Line 2
Line 3
Line 4
You'd want the "extra" indentation of lines 2 and 4, relative to lines 1 and 3, to be proportional to the font size. Of course, you might want the entire poem to be indented relative to the surrounding text, and a fixed offset would be fine for that.