Substituting service in next lower grade for raise of grade.
20. Except as hereinafter provided, an applicant who has served in a lower grade than that for which he is licensed may substitute service in the grade next below that for which he is licensed, which service shall count one-half in computing experience for raise of grade. For example, if an applicant holds chief mate's license and has served nine months as chief mate and six months as second mate, the six months' service as second mate shall count as three months as chief mate in computing experience.