I need a little help with some math please. Diluting an APA

I've got 4 gallons of 1.013 beer. If I add 1 gallon of water to it, am I correct in thinking this will take me down to about a 1.010?

I overshot the crap out of my O.G. with a whopping 83! It attenuated down to 13, but a 9% ABV American Pale Ale is a little too much for my taste.

Also, after adding 1 gallon of water, do I subtract 20% from my ABV?

Thanks a bunch!

cheers,
Zach

For gravity: (4x13)/5=10.4(1.0104) so that calculation is correct.
For ABV: 4*9=5*x --->x=.8*9=7.2, your calculation is correct again.

This beer actually came out alright. I increased the hops for all the additions, and it came out pretty well balanced. I was most happy that it doesn't have a big alcohol taste. It's masked really well with the malt and hops.

I decided to call it an Imperial Pale Ale in lieu of the Zymurgy article in the new issue. Watch out though, you can't drink too many of these at 9%.

We'll see how it tastes after another month in the bottle.