In the R Rural Agricultural District, on any sending tract that has exercised rights to subdivide or erect residences pursuant to § 301.4A, the number of available transferable development rights shall be calculated by multiplying the area of the sending tract, in acres, as it existed prior to subdivision or erection of residences pursuant to § 301.4A, times 0.25 and, from the product of that calculation, subtracting: 1) the product of four times the number of lots subdivided or residential dwellings erected pursuant to § 301.4A; and further subtracting: 2) the number of residences on the sending tract, if any, which existed prior to subdivision of lots or erection of residential dwellings pursuant to § 301.4A. As an example, if the area of the sending tract from which transferable development rights shall be severed is 100 acres, but two one-acre residential lots have been subdivided pursuant to § 301.4A, and there is one existing dwelling, there shall be 16 transferable development rights ([100 X 0.25] - [4 X 2] - 1 = 16).