Just playing around with my refractometer app, using 20° OG and 10° FG I get the following ABVs by changing the correction factor to...
1.08 7.2%
1.04 7.5%
1.00 7.8%
.98 7.8%
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?