Ajf is right. Jason is too, to an extent. You need to look at the difference between the OG and the FG, then multiple by 131.
I take it your hydrometer has multiple scales, that is how you knew that 1.020 is about 2.5% abv. This is the potential alcohol by volume.
This assumes that the difference in density between your fluid and water is due to the amount of 100% fermentable sugars, and that if all those sugars were fermented, they would produce alcohol, so you would have a water alcohol solution of 2.5% by volume of alcohol.
The difference in specific gravity is rarely due solely to the presence of fermentables, but that's a good thing right? Because these unfermentables are what yield mouthfeel, and body and malty flavors.
So you started with an SG of 1.065, your OG, which has a potential alcohol value of about 8.5% and your FG of 1.020 has a potential value of about 2.5%, so your in the neighbor hood of 6% or
1.065-1.020=0.045, 0.045*131= 5.895
I B Mongo has a good point too, temperature affects SG. The warmer the temperature, the lower the density of the fluid. Your hydrometer should say what the calibration temp is, usually 60 or 65*F. This is the temperature that that hydrometer will read 1.000 in water.
Once you know the calibration temp. you can find charts for the adjustment for a given temperature. They have different offsets for 60*F and 65*F hydrometers. For instance, my hydrometer was calibrated at 60*F and at 77 degrees the chart the came with it says I should add 0.002. So an SG of 1.020 at 77*F would actually be 1.022.
You should also check your hydrometer in your water at the calibration temperature to see if it reads 1.000. If it doesn't, you can account for this difference in your readings. My water reads 1.004 at 60*F. It could be the water or the hydrometer, but it doesn't mater, I just know my hydrometer reads 0.004 high in the my brew water, so I will deduct this from all of my readings with this hydrometer.
Using the above example, my SG of 1.022 is now 1.022-0.004= 1.018. Which for an extract, or a brew with a lot of adjuncts, could be just fine for an FG.
Hope this helps
