So if I'm understanding your question, it is "how do you convert the gravity numbers that we typically use into a percentage of residual sugar?"
The simple answer is you don't. Alcohol has a gravity much lower than 1.000 (0.787 if you were wondering), so the alcohol in the mix is a variable amount depending on the batch that throws the calculations off. A wine or mead with 8% ABV and a gravity of 1.005 has a lot less sugar than a mead with 20% ABV and a gravity of 1.005. You can get an accurate measurement by directly measuring the sugar using Clinitest tablets - but for a sweet batch, you'll have to dilute it into the range of the test (which is around 2% sugar if I recall)
With that said, you can rough out a guesstimate if you make some assumptions. Start with assuming that for a regular strength mead that a completely dry batch would have a gravity of 0.990. Based on tables like the
one at brsquared you can say that every 10 gravity points (0.010) equates to roughly 27 grams per liter (or 2.7%) of sugar. So a mead that has a final gravity of 1.000 likely has 2.7 percent, and a mead with a gravity of 1.010 would be close to 5.4%. Mind you this is rough ballpark number that is based on a big assumption (that dry is 0.990 which is not true in most cases).
The NewBee Guide uses the following scale which is not too far from the BJCP numbers:
Dry: 0.990 – 1.006
Medium: 1.006 – 1.015
Sweet: 1.012 – 1.020
Dessert: 1.020+
What I find is that most meads have a lot less acid and tannin than wines. As a result, at the same level of sugar, meads may taste sweeter than wines because the acidity/tannin masks the perception of sweetness in the wine. Thus trying to compare them at the same sugar level may not be fair if the acidity/tannin is not also matched. Meads really are very different from wines.
I hope that helps to answer your question.
Medsen
