Particularly during lautering and during a boil, I have a hard time keeping track of water/wort volume. What method(s) and equipment do you all recommend for accurately calculating liquid volume during a boil/water heating?
For most things I just use my trusty stick, otherwise known as a thin dowel from the hardware store. Pour one gallon into your pot, stand stick in pot, note water line, and mark with a sharpie or other implement. The curvature of a pot can affect the height of each tick mark, so I usually experimentally measure a few more gallons until I'm at the straight sides of the pot, then extrapolate the distance between each tick mark to mark off the remaining gallons up to 15.
This doesn't work so well at full boil, with the bubbling and rolling and general chaos going on at the surface. In that situation, the best method I've seen is a pre-calibrated sight gauge attached to the kettle. The liquid on the sight gauge tends not to boil, thus allowing you to get a decent reading. The downside of measuring during boil is hot liquid expansion, so always note that your wort will compress a bit after cooling.
For my kettle I made a measuring rod using a length of copper pipe (Home Depot sells 3' lengths of pre-cut pipe for about $2). I used a file to mark it at 1 quart increments.
When boiling, I just leave the rod in the kettle. Since it's copper, it's contributing valuable nutrients to the wort. And by staying in the boil, it's also sanitary.