I just got an email back from Kristen Huff at B-S labs. They calculate fuel dilution based on flashpoint.
We determine fuel by the flash. We know what it should be (at or above), and
if it's lower than that, we calculate fuel by how low the flash point is. I
don't know if I can answer your question about weight or volume, since we
don't measure either when we calculate fuel.
One note--calculating fuel by the flash point, while a valid ASTM test, is
not a fool proof method. There are other things, like solvent-based engine
cleaners and some types of additives, that can lower a flash point and fool
us into thinking you have fuel present, when you don't. Let us know if
you've used anything like that, because it might change our thoughts on
whether you have fuel in the oil.