The requirement to convert from one hardness test scale to another is quite common and is covered by various International Standards ASTM 14012b, ISO 18265. These standards point out that, due to the differences in the various types of hardness test methods, it is not possible to show a constant conversion relationship across all materials.
There are a number of conversion charts according to material type and it is essential to use the correct one.
Equations generated from the specific hardness numbers can be found on: www.asminternational.org