Basic facts:

2,7 grammes of sugar gives 1 hydrometer (with Oechslescale) degree in
1 litre of liquid.

17 grammes of sugar gives 1% of alcohol in one litre of wine.

Wine yeast only tolerates 17-18% alcohol, after which it rests.

A reading of over 90 Hydrometer (with Oechslescale) degrees at the start or
75 degrees during fermentation can stop the process.

Proceed as follows:

Start the fermentation (with Oechslescale) at approx. +80
degrees (4 kg sugar in 20 litres).

When 40 degrees has been fermented (the hydrometer reads 40 degrees),
add the remaining sugar (about 2 kg).