Where it is not practicable to compute the annual gross volume of sales under Sec. 794.123 or Sec. 794.124 in time to determine obligations under the Act for the current quarter, an enterprise may use a 1-month grace period. If this 1-month grace period is used, the computations made under those sections 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.