You can use your refractometer for final readings and ABV calculation. But here's the thing:
1) Brewer's Friend refractometer calculator uses the Novotny calculation, which is really only good down to 1.014 (3.5 Brix). So if your beer is now at 2.0 Brix as predicted by the software, you need to...
2) Use the Sean Terrill calculator when less than 3.5 Brix.
http://seanterrill.com/2012/01/06/refractometer-calculator/
But...
3) Terrill assumes a default WCF value of 1.04, which might not be correct for your particular refractometer. Meanwhile, Brewer's Friend assumes default WCF 1.00 (i.e., no correction) unless you know better. My recommendation is to use 1.00 until you know better. Personally on mine the factor is in fact 0.995. YMMV. So.... when you use the Terrill calculator, I recommend changing the WCF to 1.00.
Doing that, your result at 5.0 Brix is likely actually 2.6 Brix in real life, which is less than 3.5 Brix, and so then using Terrill calculator, he says you're at about FG 1.010 and 3.8% ABV. But wait...
4) Terrill's ABV calculation is wrong IMO. Instead use the standard (OG - FG) * 131 = (1.040 - 1.010) * 131 = 3.9% ABV. Slightly higher than Terrill thinks. And only good to 1 decimal point. None of this is accurate at all to more decimal points than that, for heavenssakes.
Confused? Yeah, I know..... it's a mess. But accurate, if you want it to be. I swear to all that is holy. I have spent hundreds if not thousands of hours digging into all these things because I'm an extreme math nerd. I say you can trust this as being accurate. If you don't believe me, buy a hydrometer and find out.
