A good hardness range for a katana typically falls between 58 HRC and 62 HRC (Rockwell Hardness Scale), although there can be some variation depending on the specific design, intended use, and personal preferences. Here are some factors to consider regarding the hardness of a katana:

1. Balance between Hardness and Toughness: A katana requires a delicate balance between hardness and toughness. A harder blade can offer better edge retention and cutting performance, while a slightly softer blade provides greater resilience and resistance to chipping or breaking. A hardness range of 58-62 HRC generally strikes a good balance between these factors.
2. Traditional Japanese Katana Standards: Traditional Japanese swordsmithing techniques and standards often aim for a hardness range of around 60 HRC for the cutting edge (ha) and slightly lower hardness for the spine (mune). This differential hardening allows for a hard and sharp edge while maintaining flexibility and strength in the blade.
3. Steel Type and Heat Treatment: Different types of steel used in katana blades have varying optimal hardness ranges. Traditional tamahagane steel, for example, is typically heat treated to achieve the desired hardness range for a katana. Other steel types, such as modern high-carbon steels or tool steels, can also be heat treated to attain suitable hardness levels.
4. Intended Use and Personal Preference: The intended use of the katana and personal preferences can influence the desired hardness range. A slightly lower hardness may be favored by practitioners of martial arts or for more intense cutting practices, while a higher hardness may be preferred for decorative or collection purposes.

It's important to note that while hardness is an essential consideration, other factors such as the quality of the steel, heat treatment, forging techniques, and overall craftsmanship are also critical to the performance, durability, and balance of a katana.

If you are considering a katana, it is recommended to consult with experienced swordsmiths or experts who can provide guidance based on the specific requirements, intended use, and personal preferences to ensure the suitability and performance of the blade.