What's ideal humidity

80% and 25 C

For seedlings it is around 80%

A humidity dome can be bought to cover them. Or you can make one. Most people start on solo cups and cover then with another cup but make sure it is a clear cup. Poke wholes for ventilation and mist the sides of the cup. If you most the top then drops can form and fall on the baby's thus burning their leafs. Make sure to always mist the sides. For this stage 80% but each stage of growth requires different humidity levels and its all determined by your temperature. Google a VPD chart for Autoflowers. It shows yellow and humidity ranges for each stage of growth. When the Temps go up so does the humidity level. Their directly connected. When the Temps are lower the humidity should also be lower. Ideal conditions are below with a 5 degree drop in temp at night. For seedling stage 75-80°F and 75-85% RH will work. Veg 70-75°F and 60-70% RH Flower 70-75°F and 45-55% RH

At that stage, 100% humidity. You can cover them with translucent plastic cups. Make sure the are not too close to your lights, lest they get steamed by the excessive heat. Just vent them by removing cup each day to let them breathe sufficiently

70-80% is the sweet spot for seedlings. A misted humidity dome(or a clear plastic tote turned upside-down if you don't have a dome) works well.