The board may set a ratio of apprentice plumbers and apprentice gas fitters working under the supervision of any journeyman or master plumber or gas fitter; provided, the ratio shall be not less than three apprentices to any one journeyman or master plumber or gas fitter for those individuals working on non-residential jobsites and not less that five apprentices to any one journeyman or master plumber or gas fitter for those individuals working on residential jobsites.