Formula is a bit complex...you have to calculate molar concentrations of sugar in cider and juice then how many grams of sugar cause 1 point raise in water and relate it to the juice. Then what molar concentration the final solution should be. Then formula is:
final concentration = Total Molar concentration/ Total volume
I'm in the process of putting together a spreadsheet that covers various different sugar/juice additions..
That's why I jumped on this I'll be really interested to see if this works...
I think you have to add in the ball park of 5ltrs..it depends on your exact volume of cider....
If you have 5 gallons (18.4 ltrs) you should add 4.9 litres..so maybe add 4 ltrs (just over a gallon) make sure it's mixed, take a reading...then add and take readings until you hit your required sg...but it should be just under 5 ltrs ball park....that's why people use concentrate..it would be just over 1 ltr of concentrate for the same rise.
If you could do this and let me know exactly how much you end up adding, it would help me enormously..if you don't feel like being a guinea pig then I don't blame you at all..
If I can confirm that the calculations are working I'll share the spreadsheet with you..
But, just to stress, this is a work in progress and you follow this advice at your own risk...however, you could be contributing to real -drinkable- science.
cheers