Track width = center of wheel to center of wheel? or wheel hub to wheel hub?
I have been going by hub to hub on my posts and of my design.
Then was using over all width to the widest point of either the body or outside of the tires which ever is greater.
I figured track width would be to the hubs as if you change to offset of the wheel, then your track width would change.
I believe the manufacturers and magazines measure it from center of wheel to center of wheel, but they're measuring a completed car. From a "drawing" or "designing" standpoint, any of the ways you suggest will work, as long as you're consistent. However, your last statement is true, in that effective track width changes with the wheel/tire package. I'd suggest picking a wheel/tire combo early on in your design and using that set-up to measure track width. Note that you can use different wheel offsets front/rear to achieve same effective track with different "hub widths".
Totally confused now? Good, my work here is finished...