Where it is not practicable to compute the annual gross volume of sales or business under paragraph (b) of Sec. 779.266 in time to determine obligations under the Act for the current quarter, an enterprise or establishment may use a 1-month grace period. If this 1-month grace period is used, the computations made under this section will determine its obligations under the Act for the 3-month period commencing 1 month after the end of the preceding calendar or fiscal quarter. Once adopted the same basis must be used for each successive 3-month period.