I seem to have a bad habit of mis-calculating ingredients, particularly the liquid ones. Can I just boil off excess water that I've added to a flour mixture (in this case a pizza dough)? Any other tricks, other than adding more flour (= huge amount of dough)?
If everything is already combined, you can't really boil off the liquid without also cooking the flour, killing the yeast, and ruining your dough. If you don't want the large amount of dough you would have from just adding flour to what you have, your best bet is to just throw away a portion of what you've made, add flour to the remainder, and proceed. In the future, try adding 75% of the water, then adding the rest as needed. Sometimes this is the only way to account for additional moisture your flour may have absorbed in humid environments.
In the case of pizza dough though, if you have too much, if you portion it, lightly oil the portions, and freeze them individually, they should keep fairly well.
How much extra water are you adding? If it's so much that simply adding a bit of extra flour won't do, I'd probably first looking at why you're measuring wrong. I don't imagine boiling it is a good idea. You don't want to boil your dough.
I would suggest whenever you are dealing with flour, to do it by weight. Google out a similar recipe that gives you weights instead of (or in addition to volume). Measuring flour by volume is notorious affected by humidity, how long the flour has been sitting in it's container, and numerous other things.
If you don't have or don't want to use a scale, you can follow the process found on King Arthur's site (and numerous other places) for accurately measuring flour. The short version is that it tells you to "fluff up" your flour first. If you fluff up the flour and then measure out the cup (scraping a straight line across the top of the measuring cup with a chopstick or something), it is almost always sufficient for my purposes.