I think the way non-lethal damage works is that I have 10/15 HP, and 7/10 non-lethal HP. Non-lethal damage is tallied alongside your current HP, and when it meets current HP you're staggered, exceeds and you're unconscious.
If I took 5 more HP damage, then my total non-lethal track would be 2/5 remaining (3 taken, and 2 remaining before stagger, 3 remaining before knockout).
This is what I have running on a notepad on my desktop at work and home, but I can edit it depending on your ruling for the way it works.