calculating % alcohol

so i've brewed three batches in my infant career as a homebrewer and the first two were kits so i they came with an estimated alcohol percentage. my third batch was bottled last night and is a custom recipe, stolen from the database, modified slightly. my question is, how do i calculate the percent alcohol the beer should roughly be?

info:

beer: cherry ale, used 8.5 lbs of cherries in primary
OG: 1.064
FG: 1.020
primary: 10 days
secondary: 20 days

and i believe i'm looking for percent alcohol by volume, not weight, but obviously i can get from one to the other if need be.

ABV = 1.31 x (SG-FG)

ABV = 1.31 x (1.064-1.020)

ABV = .05764 = 5.8% = nice!

I use this online calculator. Pretty simple.

thanks guys,

i knew i was going to be above 5, but i was hopping for 6.5ish, still, 5.8 is good. now i wait to see how it turns out

Ok, so it looks as though I may have screwed up my second batch (at least alcohol-wise). Of course, being a moron, I only took the gravity readings and not the temperatures of the wort (even after I knew it was important...). For my raspberry wheat, I had a SG of 1.038 and a FG of 1.018, which calculates to 2.5% abv, which seems... wrong. I can't find the paper that listed what the gravities should be, but I remember the starting was maybe +/- .002-4.

So, am I correct in that these are the possibilities?

1. My beer has an insanely low abv (if it tastes good, I don't really care)
2. I suck at taking hydrometer readings/adjusting for temperature (I like this one, as it means it's ok)
3. I bottled before fermentation was done and can expect some lovely bottle bombs in the future (I didn't take a reading a few days before bottling because airlock activity had stopped 7-10 days earlier, and I figured 2 weeks is plenty for a wheat; was my desire to save the wort misguided?)

Gravity for extract brewing is pretty constant as long as you don't adjust the volume. How much extract in the latest batch? You might consider testing your hydrometer. It should read 1.0 in water at 60 degrees. They have been known to screw up.