If you look at your hydrometer, an Original Specific Gravity of 1.076 gives you a Potential Alcohol of about 10.25%. Take the starting potential alcohol number & subtract the final potential alcohol number to get your actual ABV.
If your wine completely stops fermenting at 1.000, your finished wine will have an ABV of 10.25%:
10.25 minus 0 = 10.25.
If your fermentation stops at 1.020, your final ABV would be about 8.25%:
10.25 minus 2 = 8.25%
Once your SG gets below zero, you add the number in the potential alcohol column to your starting potential alcohol number:
If your final SG stops at 0.994, your ABV would be 12.25%:
10.25 plus 2 = 12.25%.
Your yeast will work on eating the sugar in the must every day, and that will lower that number a little bit every day. The specific gravity measures the amount of sugar in your juice. The longer your yeast eats the sugar (producing alcohol & carbon dioxide as byproducts), the lower that number on the hydrometer gets, and the higher the actual alcohol level in your wine gets.
The first thing you want to do when making a wine is to check the SG of the juice, before you start adding yeast or chemicals. If the SG is not high enough to give you an ABV that you desire, you add a measured amount of sugar to raise the SG to the proper level. For dry reds, I prefer 12.5 to 13.5% (SG of 1.095 to 1.100). For sweet fruit wines, I prefer 10.5 to 11.5% (SG of 1.076 to 1.086).
Besides the SG, there are a couple of other items you should check, before adding acid blend, etc. The first is the PH, and the second is the Total Acid Level. A PH test kit can be found for around $5, and an acid test kit for around $10. A lot of recipes call for adding acid blend, but the acid level really should be checked before adding any acid. The reason being is that in different parts of the country, where you have hotter or colder climates, the acid level in the exact same fruits will be completely different. Here in the Midwest (Indiana) our fruit tends to have a higher acid level than in Texas. For example, Jacks Cherry recipes call for 2 TBLSP of acid blend, but with my higher acid levels in my fruit, that would be too much.
With the PH level, you want a fairly low PH of 3.0 to 3.6 for a wine that will age in the bottle, because a lower PH will prevent spoilage critters from attacking your wine in the bottle and turning it into vinegar. A wine with a PH of higher than 3.6 should be drank fairly young, before it has a chance to spoil.
