The 1.4 times rule is a rule of thumb. Of course it only works when the inlet area is the right size for this rule to work. A larger inlet might work well with a smaller outlet, and a smaller inlet might work with a larger outlet. The speed of the aircraft matters also. For example with full scale planes fast planes will have small inlets with large outlets and cowl flaps for low speed. This works well but the problem with a small inlet is getting the fresh air to diffuse over the entire engine and not pass most of it over a small area, or short circut the engine altogether. Using a large plenum area over the engine with the tight baffles usually works to prevent this. Because of this the lower speed planes like C 172's will usually have larger inlet areas of the smaller planes.

Because our models have less extremes in their flight envelope, that is there are few really fast planes. The rule of thumb should work well with the right inlet area.