I know this is a subject of potential consternation for women. Since it's so context-driven and tact is a new concept for me, I've come up with a few hard and fast rules.

When I'm discussing a woman with my wife, we have our own protocols.

Women from the south and certain black women are ma'am.

If I am speaking to a group of women, they are y'all.

If I'm speaking to a woman I don't know, or who doesn't meet any of the above criteria, I avoid the issue entirely. I find "thank you" works just as well as "Thank you ma'am."