Kim, if you stick the handle of a wooden spoon into your pot and make a line with a sharpie at the liquid level, then it's very easy to figure out when you've boiled off about half -- just use the spoon handle to measure.
Janet, that's brilliant. I might even forget about the Sharpie and just lay the spoon aside and use the color and stickiness as a guide. Or use a cheapo chopstick or long cheapo wooden skewer.
I have a very small rubber band I keep on a wooden spoon for this purpose...I just move it up and down as needed. If you know someone with braces ask them for a couple, they are about the size of a cheerio.