Every brew is a bit different so one way to estimate the difference is to compare your refractometer with a hydrometer for that particular solution. Most refractometers have been designed for measuring sucrose solutions unfortunately maltose has a slightly different refractive index (and wort contains some other non-sugar bits that alter the solution's refraction).

910g of water), we would have 9% sugar by weight, that is 9°P, or equivalently 9☋x. So if we have a 1kg solution that we know contains 90g of sugar (i.e. These scales correspond to the percent of sucrose in a pure water solution (by weight). Traditionally brewers use the Plato scale and vintners use the Brix scale. For most purposes, Brix and Plato are interchangeable e.g. the Brix scale is based on the Balling scale (other scales include Baumé and Oechsle). There's lots of history as to how these scales were developed and evolved e.g. The units often used to indicate the amount of sucrose in a liquid are degrees Brix (☋x) or degrees Plato (°P). The good news is that the index of refraction of a liquid is directly related to its density (affected by, for example, the amount of sugar in the liquid), which in turn can be used to calculate its specific gravity. However, a refractometer actually measures the index of refraction of the liquid, not the specific gravity. Also, most refractometers have an Automatic Temperature Compensation (ATC) so no correction needed.

So why use a refractometer? First of all, refractometers are great because you only need a few drops of sample liquid which reduces waste and risk of contamination.

Plato hydrometer temperature correction