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.