Thanks - bookmarked that oneNo need to get too complicated with the refractometer. When I take my OG prior to fermentation with the hydrometer I also note the reading from the refractometer. Enter these both in a free online calculator and this will give you a calibration figure of close to 1.00.
Every time you take a new refractometer reading once alcohol is present just enter the OG reading, new reading and calibration figure.
One such calculator is here:
https://www.brewersfriend.com/refractometer-calculator/