You're perfectly right. The so called "hardness conversion tables" are only approximate and are to be used to make a rough estimate, not precise calculations.
There are plenty of tables around, some serious and some other not. By "serious" I mean a table that warns the user stating what I have said before, i.e., that there's not an exact equivalence between hardness numbers (Brinnell, Vickers and Rockwell).
Now, I agree with you that sometimes it's necessary to make that conversion, even if it's not precise. During my professional life, i.e., before becoming a college professor, I ran many times into that situation. In those cases I used a table I had confidence in.
For sure, one of the most dependable tables is that issued by the Society of Automotive Engineers, which you'll find in the SAE Handbook. Now, having to open the SAE Handbook every time you have to make a conversion is a little annoying, so you better manage to get a good table for daily use.
Here at the Metallurgical Lab of Mackenzie University we use a table issued by ACCO (American Chain and Cable Company), which we hung on the wall.
I'm almost sure that there's also an ASTM Standard Conversion Table. I'll take a look at the ASTM books in our library and get back to you.
Giovanni S. Crisi
Sao Paulo - Brazil