Your best bet is to calibrate and mark your fermenter. Use a 1 gallon bottle and use a measuring cup to mark quarts on it, then use the one gallon bottle to fill your carboy 1 gallon or less at a time. Mark on the outside of the fermenter with a Sharpie. With a carboy it's pretty straightforward, but it's still easy with a bucket - the plastic is translucent so you can see the liquid level if the light is bright (if your room light isn't bright enough, use a flashlight).