I just brewed this as my first AG attempt. I'm not efficient with all the math at this point but my efficiency wasn't great. I hit my temps for the mash, 150 for 1 hour. I used a 5 gallon water cooler MT and did a batch sparge but I think I lost my efficiency by not waiting long enough during the sparge for my 2 second runoff. (I hope I have my terminology right). My pre-boil gravity was only 1.032 when I adjusted for temps. After some reading I decided to try to adjust for the low gravity by adding some DME. Not wanting to overdo it I added 1 lb of extra light DME but then forgot to take my gravity reading again before I pitched my yeast. Hopefully I got it close enough to still make it good. I brewed on Tuesday and by Wednesday evening I had some activity, this morning my airlock is rocking away. Hopefully this turns out as good or better than the 3 extract batches of this I've brewed.
I doubt you loss efficiency on the batch sparge since you sparge with hotter water to basically stop the process. When you batch you just fill with water, stir like hell, vorlouf and drain. The conversion is already done when you reach that point. You're just rinsing the grain bed of the residual sugar.
I would evaluate the crush, or verify water levels.
If you added DME you expect about 1.040 per lb per gallon. So you raised your gravity about 1.040/5 since you diluted it down by 1/5th. So it would be about 1.008
So you should be about 1.039 (right on the money for this recipe)
I THINK my math is right, but someone mightier than me will correct if wrong.
Btw, assuming you ended up at 5gal, your efficiency was about 50%, so ya I'd evaluate a few things to determine what happened. When you did your batch sparge, did you add the amount of water you needed to hit the volume or did you just fill the tun?
Edit: I just noticed you said your PRE boil was 1.032. How much extra volume did you have and whats you're boil off rate? Depending on that, you very well could've hit target since 1.039 is the target.
Assuming you had 1.032 at 6 gallons and you boiled off 1 gallon during the boil, you're at about 1.038 (just shy of target) add your 1lb DME and you're sitting at around 1.046.
With your 1lb DME addition, you're definitely over target. But this is such a low ABV beer. Next time I make it, and I will, I'm going to aim for closer to 5/5.5%
If you finish at target FG, you can expect about 4.99% ABV right where I'd want it.