The basis of compensation and benefits to be paid to a volunteer firefighter, an emergency or disaster volunteer, volunteer health practitioner, or a community emergency response team member under the terms of this chapter shall be determined in accordance with the provisions of section 65-05-09; provided, however, that the average weekly wage of the claimant shall be determined from a computation of income derived from the claimant’s business or employment for which coverage is required or otherwise secured at the date of first disability.