Hi Masshole1388, You ask how much apple juice you should add. That depends on how sweet you want the cider. And THAT might depend on the level of alcohol , the acidity of the cider, how much tannin is in the cider/apple juice, and how much sugar is in the juice.
Really, (IMO) the only way to know how much juice to add is to do a bench test. Take a few identical measures of the cider and add to each measure a known but different volume of juice. Taste each sample. You may have to repeat this several times to find the er.. um... sweet spot but when you find the sweet spot you simply multiply the volume of juice you added to that glass by the number of glasses that would make up the volume (say your total volume was 20 liters (or 20000 ccs and your glass was 50 ccs then 20000/50 = 400 and if you added 10 ccs of apple juice then you will need to add 400 * 10 (or 4000 ccs or 4 liters of apple juice) to make the whole batch as sweet as that glass in the sweet spot.
Alternatively, if you prefer to take a sledge hammer to this, ask yourself how sweet you like your cider? Do you prefer your cider to have a gravity of 1.005 or 1.010 or 1.015 or ? You decide. Then measure the gravity of the cider (may be around 1.000) and measure the gravity of the apple juice (could be about 1.040 - 1.050) . You've made 5 gallons so each gallon of apple juice will raise the volume by one gallon - and you need to take that into account, but each gallon will also raise the gravity of the TOTAL by say, 45 points-divided by the new total volume (45/6 = 9 points; 90/7 = 12.8 points; 135/8 = 16.8 points )... At the same time you will be diluting the ABV because you are increasing the volume of liquid with the sugar but that liquid has the equivalence of water ... So one alternative is to add concentrated apple juice to sweeten your cider...
