Dew point
Your hygrometer gives temperature and humidity: here you get the number that matters — below which surface temperature condensation forms.
What the dew point is
It's the temperature at which cooling air reaches 100% humidity: below it, vapour condenses into water. At 20 °C and 60% humidity the dew point is ~12 °C: any colder surface (panes, external walls, corners and thermal bridges) gets wet.
Why mould
Mould doesn't need droplets: a wall whose surface humidity stays above 80% for long is enough. It typically happens behind furniture against external walls, where air doesn't move and the surface stays cold: that's why this tool reasons about surfaces, not just air.
What to do
Three weapons: ventilate (5-10 minutes with windows wide open, 2-3 times a day, especially after showers and cooking), heat evenly (an always-cold room is a room at risk) and pull furniture a few centimetres away from external walls. A dehumidifier helps when humidity stays above 60-65%.
Note
Magnus formula (Sonntag coefficients), the same used by professional data loggers: within ±0.3 °C in the domestic range. For extended or recurring mould you need a proper diagnosis (thermal bridges, rising damp): call a specialist.