We typically do not have mixed light commercial and residential fed from the same transformer, even if they are adjacent to one another.
Typically, this formula is used for residential or small retail situations such as a "strip mall" or small building with 5 or 6 suites. For applications larger than that we use a Demand meter which records the actual peak demand of each consumer.
OK. This makes a little more sense.
What you call small retail, or small 5-6 suite building, I would consider small commercial . . . this is just a jargon usage. (Toe-may-toe; toe-mah-toe)
In the early '70s I was involved with a project to extract alerts for possible transformer bank overloads from the monthly billing data. The computer billing system was extremely limited, so most attention was paid to the demand metering readings.
Given the changes in the load profile of a "typical" residential dwelling, I would submit that the curve that your predecessor's formula attempted to "fit" has probably altered a bit.
The effects of today's switching power supplies, used in CFLs, computers, video equipment, gaming systems, low voltage lighting, fluorescents, offers a reactive component unlike anything in the almost entirely resistive / inductive load of the Sixties.