Refractometers are one of the most misunderstood tools in home brewing. The two biggest mistakes are using the sg side of a dual brix/sg scale (the conversion is non-linear, and the scale will likely become increasingly off between the two the higher the gravity), and using one like a hydrometer, ie simply subtracting final from original gravity. Instead, only use the brix side of the scale, and use a calculator such as this one to determine the actual specific gravity/abv once fermentation begins: www.brewersfriend.com/refractometer-calculator/ (don’t worry too much about the Wort Correction Factor.) Do that, and your refractometer will be your best friend.