Channel spacing and positioning has always been down to manufacturers’ recommendations, perhaps because the different proprietary products available make it difficult to provide national guidance. We were the first to work it out analytically, based upon deflection and ultimate strength of the various channels with each possible substrate, but had to provide separate data tables (built into the software) for each product. The default 150mm margin we took from one company’s recommendation on what is common practice, as it was not important structurally (but very important for erection and serviceability for the reasons you mention). In the User Guide where we show how to change it, we use 75mm as an example of what it might typically be changed to.
This is the same software your sign manufacturer was using, as SignLoad is built into SignPlot, but they must have changed the channel margin to 200mm for reasons known only to themselves.