<font color="blue"> But would not a better formula be the number of bathrooms? </font>
I agree that the number of bathrooms seems logical. However, there is generally a correlation between the number of bedrooms and the number of people who are living in the house. And, it seems to me that the number of people living in the house is a more accurate indicator of the load the family is putting on the septic system than the number of bathrooms into which they are generating the load. In other words, 7 people are going to gerate more waste than 3, whether they are doing it in 1 bathroom or 3.
Just my thoughts.