For the purpose of making assessments of premiums to be charged against municipalities for protection of volunteer firefighters, emergency or disaster volunteers, volunteer health practitioners, and community emergency response team members, the organization shall make such survey as may seem advisable to ascertain the probable annual expenditures necessary to be paid out of the fund to carry out this chapter, and shall fix the annual charges and assessments which must be made against municipalities employing volunteer firefighters, emergency or disaster volunteers, volunteer health practitioners, and community emergency response team members. The charge must be a fixed sum for each one hundred of the population of the municipality involved and uniform as to all such involved municipalities but in proportion to the population of the municipality. In determining the amount of premium charge, the organization may apply the system of experience rating provided in this title, as applied to other risks. The organization may establish a minimum charge or assessment to be applicable to any municipality for which the fixed rate or charge multiplied by the number of hundreds of the population of the municipality would amount to less than the amount of the minimum charge or assessment. The population of a municipality shall be that shown by the latest official North Dakota state or United States government census, whichever may be the later.