When I took a quick look at ASTM A-500 I did not see a hardness test requirement, so you have the option of just using the approximate relationship between hardness and tensile strength if you are trying to use hardness to estimate strength. Tables of conversion between hardness and tensile strength are common. Conversions between yield strength and hardness are less common and are based on empirical data, generally include a lot of conservatism in the conversion, and are most accurate when determined using data for steels of the same or similar grade and manufacturing method of interest. A-500 grade B has a specified minimum tensile strength of 58,000 psi. A common conversion of hardness to tensile strength results in a hardness of about BHN 112 (measured with 3000 kg load) for that tensile strength.

The "rule of 500" applies to carbon and low alloy steels for finding the Brinell hardness number. The BHN multiplied by 500 is approximately equal to the materials tensile strength. Tensile strength divided by 500 is the approximate BHN. It is approximate so maybe it should only be a "rule of thumb".