When the must begins fermentation the sugar content is high. The hydrometer (with Oechslescale) reading is then about +80 degrees.
As the sugar ferments to alcohol the reading gets lower and lower.

Measure the gravity of the must at the start of fermentation.

Refer to the table "When the wine has fermented out" to see what the hydrometer (with Oechslescale)  reading should be.

Measure the wine during fermentation.

By seeing where the reading stands between the start value and final
value you can see how far fermentation has progressed