Calculating bottling volume

I understand the amount of priming sugar used should be directly related to the style of beer and volume of beer being bottled. How do you figure out the volume of beer prior to racking into the bottling bucket (since the priming sugar should already be in the bucket)?

For example, I recently brewed a wit (5.5G plus a 1.5L starter). Obviously, some of the volume at bottling time will be made of break, yeast, etc and not to mention I won't be able to rack over every last ounce of beer in hopes of avoiding the trub. Once it's done fermenting how do I come up with my bottled volume? Do you just guess?

Next time you aren't using your carboy, you could fill it with water a half gallon at a time and make marks on the outside of it. That way you can see the total volume in it as well as the volume of the trub.

^^^ THAT.

All of your equipment should be calibrated, not just thermometers and hydrometers.

That would make complete sense, but I'm using a bucket. I can certainly see through the bucket a bit, but I don't think I'd be able to determine where the trub ends and liquid begins (although I've never actually tried to look).

I always figure I'm going to lose 0.25-0.5 gallons between my fermenter and my bottling bucket, depending on the amount of trub and how compact it is. Because my fermenting bucket has gallon marks on it, it's easy to estimate how much will end up in the bottling bucket.

But then again, a half gallon is only going to make the difference of a couple of tenths of volumes of CO2, which is always close enough for me.