(1) If the injured employee shall have worked in the employment in which he was working at the time of the injury, whether for the same or another employer, during substantially the whole of 13 weeks immediately preceding the injury, his average weekly wage shall be one-thirteenth of the total amount of wages earned in such employment during the 13 weeks;

(2) If the injured employee shall not have worked in such employment during substantially the whole of 13 weeks immediately preceding the injury, the wages of a similar employee in the same employment who has worked substantially the whole of such 13 weeks shall be used in making the determination under the preceding paragraph;

(3) If either of the foregoing methods cannot reasonably and fairly be applied, the full-time weekly wage of the injured employee shall be used;

(4) If compensation is due for a fractional part of the week, the compensation for such fractional part shall be determined by dividing the weekly compensation rate by the number of days employed per week to compute the amount due for each day;

(5) If the injured employee is a volunteer firefighter included under this chapter for volunteer fire-fighting services rendered to a county or municipality of this state or a volunteer law enforcement officer included under this chapter for volunteer law enforcement services rendered to a county or municipality of this state, such firefighter's or volunteer law enforcement officer's average weekly wage shall be deemed to be the Georgia average weekly earnings of production workers in manufacturing industries for the immediately preceding calendar year, as published by the Georgia Department of Labor;

(6) The average weekly wage of a member of the Georgia National Guard or State Defense Force serving on state active duty pursuant to an order by the Governor shall be the greater of:

(A) Seven-thirtieths of the monthly pay and allowances of the individual at the time of the injury, computed in accordance with Code Section 38-2-250, adjusted from time to time for appropriated increases in such monthly pay and allowances, excluding longevity increases; or

(B) If the injured member of the Georgia National Guard or the State Defense Force worked at the time of the injury in any employment other than serving as a member of the Georgia National Guard or the State Defense Force, the average weekly wage of the individual in such other employment as determined pursuant to paragraphs (1) through (5) of this Code section or, if such individual worked at the time of the injury for more than one employer, the average combined weekly wage of the individual in such multiple employment as determined pursuant to paragraphs (1) through (5) of this Code section.