Hydrometer question

I am trying to move to hydrometer readings to verify that my brew is done instead the "it's been more than three weeks so it's time to bottle" method. I have seen posts that say wait until you get the same reading three days in a row, or until you are with in a few points of your expected final gravity. My question is this, what counts as a few points? If you don't reach your expected Original gravity, how do you compinsate for that in your final? The batch I have going now was supossed to have a OG of 1.065, when I added my top off water (I'm brewing extract with specialty grains) and took a reading I only hit1.054. Expexted FG was 1.010 and today I was at 1.017. I am obviosly kinda new at this and looking for some help. Any that you can offer would be greatly appreciated. Thanks.

Many variables will affect your OG and FG (temperature, brewing technique, etc.) This is why it is recommended to check your hydrometer a few times. Once the gravity is no longer dropping, fermentation is done. In your case, you didn't hit your original gravity (maybe too much water or some other minor mishap) and then I would guess your fermentation conditions were less than idea so your yeast did not fully attenuate (ferment to their full potential). Keep up the good work, and I am sure your beer will be just fine, there is always room for improvement, that why this is such a great hobby! Keep, reading on here and try to make your brewing techniques better and your beer will continue to improve.

A few points is 2-5, depending on the target gravity. The range can be calculated based on the original gravity and the attenuation range of the yeast. With an expected OG of 1.065 and a TG of 1.010, the attenuation would be 85% (which is really high). So, starting at 1.056, your new TG would be 56(1-.085) or 1.008. Using a more realistic 75%, you'd get 1.014.

Getting 85% attenuation for an extract batch isn't very likely, even with Pacman yeast.

Thanks for the feedback and info. I think the top off water is where I went wrong. I took a gravity reading of the boil, and it was actually higher than projected. I expected 1.108 and got 1.137 then after I filled to the 5 gallon mark on my carboy I was below my expected OG.

I used white labs Irish ale yeast, attenuation is 69-74%, so sounds like your assumption is pretty close. Out of curriosity are FG and TG the same? And how do you figure out those numbers?

Thanks again.

Final Gravity and Terminal Gravity are synonymous.