Read in my copy of the GUNS of the ROYAL AIR FORCE 1939-1945, by GF Wallace some details of the Brit’s experience and decision making on the 1-4 vs 1-5 vs random tracer placement. Early on, their belt loading machines were not set up to place specific rounds at specific points in the belt, resulting in the potential for 25 tracers or some such, in a row. Tinkering and new machines advanced the process of placement and they settled in a 1-4 or 1-5 ratio. Not much more science to it than that. It’s what the machine was designed for and specified to do. Simpler than trying to make a setup that would allow adjustable ratios.
Another topic of note was the finding that aerial weapons, guns in particular, did not make it much past the 1000 round mark before it or the plane, mostly the plane, was shot down. That detail allowed them to lighten up on the engineering of the guns, making them cheaper, and removed the general 10000 round life called for in the specs.