BHN is a direct measurement of the properties of the material.
I made my own tester and it functioned well enough. The principle is simple enough and the results are what they are.
Pure lead may be an exception, but in general I would be much less sure that any given "certified alloy" has a specific BHN. Material properties involve both the chemical composition and things like grain structure. Also "trace" amounts of materials other than the primary constituents can sometimes affect results.
So, be "happy" if you have gotten better at reducing uncertainties in your measurements. However, do not expect your measurements to tell you much more than the hardness.