Optimal % humidity for growing hostas from seeds depends on the stage of growing.
Most people grow hosta seeds probably in 100% humidity on top or under 1/4 inch growing medium in a cell pack under a closed dome or in a plastic bag. I spray surface of the medium with water and have 1/2 inch water in the tray the cellpack is sitting in. When plants touch top of the dome I first lift the dome a little (pencil wide), then after a few days remove the dome. My trays are in a pseudo chamber consisting of two walls and old towels closing it in on the 2 other sides. I grow under 24 hours light using fluorescent lights to heat the surface. I use sterile seed growing mix plus perlite and a little fungicide in my water. I do not see fungus.
Bernd

Thanks Bernd! Lot of great information there for me.

Peter Kelley
St. Paul, MN USA