OG calculation

So I brewed for the first time today. I bought a kit from my LHBS and cut it in half to make a 2.5 gallon batch. I mistakenly took the OG reading from the brew pot before I topped the volume off to 2.5 gallons. My reading was 1.080 and I added an additional 0.5 gallons of water to get my final volume before pitching my yeast. Is there any way to calculate what my actual OG is with my new volume? I didn't know if I could take a reading after the addition of the yeast. The kit states that the OG should be around 1.061. Any help is greatly appreciated!

[(2gal x 1.080)+(0.5gal x 1.000)]/2.5 = 1.064

C1*v1 = c2*v2

80 * 2.0 = ? * 2.5

? = ((80*2.0)/2.5 = 64

1.064

That's great! Thanks so much.