If you start with 4.5g at 1.064, you have 288 gravity points. That's 4.5 (gallons) * 64 (the meaningful part of 1.064)
Now you add 0.5g water (with an OG of 1.00) so you have 5 gallons, but the same number of gravity points, so divide 288 (gravity points) by 5 (total gallons) and you get 57.6 Convert that back to specific gravity, and you get 1.0576
Consider a gravity point as (the specific gravity - 1.000) * 1000 if you want to be mathematically correct.
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