Gravity question

[WhoDatDad78]

I made SD-SLIM ' S blood orange hefe two weeks ago and added some LME I had laying around to boost the ABV a little. I went through the whole partial mash process and cooled my wort down to 60°F, added to my primary. It was boiled down to 4 gallons of a 5 gallon batch. So I took my OG BEFORE topping off with a gallon to bring it to 5 gallons (I'm a knucklehead, I know). My question is what would my true OG be if my reading before the top off was 1.080? And more importantly how to arrive at that determination?

 

[ncbrewer]

(1.080-1)*4/5 = 64. OG is 1.064. That's if your volumes are accurate.