I am unfamiliar with this rule of thumb. There is much that determines maximum allowable span such as the spacing of joists stated by inches on center, grade and species of wood, floor load, etc.

My rule of thumb is the maximum allowable span for a 2×10, 16″ on center, #2 yellow pine (this is the typical floor joist system), is 16′-8″. #1 yellow pine spans more. A 16′-8″, 2×10 floor span is a bouncy floor even though it meets the 40 pound per square foot, live load requirement. I like 2×10’s to max out about 16′ for performance. In a dining room floor with a china cabinet …. the 16′ span will still result in noisy china shaking and clattering as one walks across the dining room floor.

Again, the goal guides the design. Our Architects and Carpenters at Mosby Building Arts use span tables printed by the lumber organizations that reduce this selection to a quick reference. These books are available at most libraries.