I use buckets when I want to know my volume out of a batch because it is easy to get exact volume by measuring the inside height and diameter of the bucket. Pour raw beer in and measure from the top of the bucket to the raw beer and subtract from your original height. Then plug in your numbers to the volume equation and you have your overall exact volume.
V = pi * d^2 * h / 4 You can plug this in on Wolfram alpha if you do not have a calculator. You can click on extended keyboard if you just want to enter the symbol for pie also. http://www.wolframalpha.com/
I don't bother with my glass carboys. The slap on decals are made for which carboy? What inside height and width is it based on? There is suck a variety of carboys as far as height and width. The only way to get an accurate marking is by pouring a known volume of liquid in increments and then mark the outside. If I had the motivation to do that I'd probably permanently etch it on the carboy.