The most optimal humidity for psilocybe magic mushroom growth is around 90%. Different periods of the process might require higher or lower humidity levels to have the most optimal results. Maintaining a stable 90% humidity inside the fruiting chamber is a great start.

Does humidity have an effect on the strength of the magic mushrooms

No, besides that the mushrooms can grow bigger they won’t become stronger. The strength of the magic mushrooms is decided by the strain you choose. As the Golden Teacher and the Hawaiian Copelandia are one of the most strongest strain available.