I think I speak for most, but when I set up a tank and to the ratio thing it was by volume.

HCA normally recommended by volume, as its easier to measure out a scoop then it is to weigh a sample!
However, I'm a firm believer that the ratio mix isn't a big deal; its the substrate depth that will be more important. True some crabs seem to dig in a certain area (depending on ratio, moisture and even temp) but honestly most will just dig down (anywhere) until they find their "happy place" to moult.