Honestly, just trial and error. When you and @GIMN first started using classes for metrics I liked them, but I've always preferred text so was messing around trying to get them to work for me.
I've only recently come back to skinning, but I really don't have the skill of you and others so my progress is quite slow.
Main problem I ran into on this was numbers in text classes don't work by themselves. You'll notice I have a space in the text field.
I've had a very brief mess round this evening, but I didn't manage to get attributes to work. If you can would you mind sharing that with me please.