Just playing around with my refractometer app, using 20° OG and 10° FG I get the following ABVs by changing the correction factor to...
Frankly that's totally close enough for me! My main use of FG readings is to verify Terminal Gravity. When people ask me what the ABV of my beers is, I usually ask "what does it seem like?" Their perception is more valuable to me than what it actually is. Then I'll tell them a rounded off figure, like if it's calculated to 7.35% I'll say "it's about 7%".
Will using an arbitrary correction factor cause my refractometer final gravity readings to bounce around, so long as I use the same factor?