Correct gravity and fix beer!

Just brewed a no-chill fest bier yesterday and somehow missed my volume and gravity by a lot. Supposed to have 6g post boil and got 5gallons instead. And my gravity is off in the same way - 1.069 instead of 1.053 like it was supposed to be.

So, without just adding a gallon to get my volume up to what I had planned how do I know how much to add to get the gravity right? I could just add a bit at a time I guess.

And being a no chill it is sitting in a cube and waiting on the yeast. So is it ok to add mineral water without boiling and chilling?

Measure your gravity points. GP=SG * volume

You've got 5 gallons at 1.069, so 69 * 5 = 345 GP

Since GP = SG * volume, then GP/SG = volume

So 345/53 = 6.51 gallons

and that is your target volume. Add a gallon and a half of germ-free water and you're right on the money.

Sweet! That's what I needed