Each employer's base rate for the twelve months commencing January first of any calendar year is determined in accordance with Section 41-31-50 on the basis of his record up to July first of the preceding calendar year, but no employer's base rate is less than two and sixty-four hundredths percent until there have been twelve consecutive months of coverage after first becoming liable for contributions under Chapters 27 through 41 of this title. Each employer who completes twelve consecutive calendar months of coverage after first becoming liable for contributions under the chapters during the current calendar year shall have a base rate computed on the basis of his record up through the next occurring June thirtieth, with that base rate being effective for the next calendar year beginning in January.