I also don't find meassuring by weight to be any more precise than by volume, and just rather much more convenient, as zoidMan previously stated. Personally, I would go crazy if I wanted to bother with all the individual densities of 100's of flavours, from perusing MSDS's or weighing them myself. Well, of course it's doable, but I just really don't see the point at all, since your recipes will not be any different whatsoever(end-result), as the botboy quote posted by DaveP previously correctly states. I also just use a constant value of 1, though any number will work the same, except the first time trying someones elses recipe found online, but to me is irrelevant as that is to me only a "tester" anyway, to see which needs to be down or upscaled anyway. We're after repeatebility, or atleast I am, which then makes absolute precision unneeded/pointless...