It’s still fairly simple; you could do a three point calibration at each temperature. Make an additional calibration solution that is a higher specific gravity, such as 1.080. Dissolve 40 g of sucrose in 160 g of distilled water to make a 1.080 solution. While a 1.040 solution (20 g of sucrose in 180 g of distilled water) will equal 10 degrees Plato, the relationship between SG and degrees Plato is less linear the further away from 1.040 you go, so that a 1.080 SG solution is not 20 degrees Plato but rather approximately 19 degrees Plato. See here:
You certainly could wait for somebody else to do the work, but the results most relevant to your brewing situation would be your own. I would do it myself, but the extent of my outdoor brewing is limited to only having to do the boiling on my deck, with the burner conveniently located right outside my kitchen’s patio door. All of the other work is done in my kitchen, so no cold garage for me! Therefore, all of my refractometer measurements are done inside my house at typical room temperature.
