Well, there is, of course, the titular stuff vs. the emotion stuff. Primary vs. secondary is easy for the former, but I agree that the emotional stuff is harder.

Being the analytical/mathematical sort I am, I've often toyed with developing a scale. I am reminded of how, ages ago, when mrstickman would suggest playing Morton's List at any opportunity, learning of one particular game from the book. Each person makes a list of their moral rules, from least to most important. They then spend an hour starting at the top of the list and breaking rules.

I think of love in similar practical terms: love is what leads me to do things that I would otherwise consider unworthy of effort and/or wrong. From that perspective, your bullet-taking measure is spot-on, I think: it's recognizing how far you're willing to go for someone, and recognizing that such a thing is probably the truest measure you can provide of your depth of feeling for them.