I've been investigating the best way to assure proper bottle fill and have been convinced from what I have read here in the forum to fill by weight. That means, of course, knowing what 750 ml of spirit should weigh in the bottle, leading me to research a good density or specific gravity by %ABV chart. There is one I have seen from several sources that shows SG at every %ABV at 60 degrees F, with some sources including a 3 step temperature adjustment table. I then had the bright idea to use TTB Table 4 a a base, converting the "Wine gallons per pound" on the table to grams per milliliter using the conversions of gallons to milliliters and pounds to grams. I assumed the outcomes would be the same but they are not. For 40% ABV, the handy chart gives an SG (which by definition is grams per ml) of 0.9518. Converting Table 4 for 80 proof gave: 1/ (0.12616 *3785.41 ml/gal / 453.592 gm/lb)=0.9498. OK, the difference is 0.2% but I would have though density is density.
Any wisdom on a) Where I went wrong on the assumptions or calculations, b) Does it matter given the pretty loose tolerances for fill, and c) What source is commonly used?