The higher the alcohol strength, the more difficult refractometers are to read, as the scale bars move closer and closer together. From 85.7%abv the reading on the refractometer even decreases again. That is why the scale on handheld alcohol refractometers only goes up to 80%abv. But you can calculate the range above from the displayed value. You just have to know for sure if the alcohol strength is below or above 85.7%. For example, a displayed value of 70%abv can also mean 96%abv.
Since there are no scale marks above 80%abv and they would be too close to each other to read well, the range of 80-90%abv is unfortunately not measurable in practice with a refractometer, despite this calculator. But the often important range of the azeotrope can be measured with the help of this calculator.
However, if you have a precision balance, it is more accurate to dilute the alcohol before measuring and thus not use this calculator. How to do it best and a calculator for it: Refractometer Temperature Correction Alcohol Solutions

General Information about Refractometers