The provisions of chapter 143 notwithstanding, one-half of the Missouri taxable income attributed to an approved retained business facility that is earned by a taxpayer operating the approved retained business facility may be exempt from taxation under chapter 143. That portion of income attributed to the retained business facility shall be determined in a manner prescribed in paragraph (b) of subdivision (9) of section 135.276, except that compensation paid to truck drivers, rail, or barge vehicle operators shall be excluded from the fraction.