Engine displacement alone does not determine how hot or cold of a plug you need. In fact, it's one of the less important factors. The compression, fuel, exhaust system, and expected RPM of the engine matter more.

Most sport engines like the OS FX line do pretty well with a wide selection of plugs, espeically after a little run time. Higher RPM, high performance engines with high compressions frequently need colder plugs to avoid pre-ignition. Man 4-strokes need a hotter plug to help maintain combustion at lower RPMS with combustion every other rotation.

It is quite possible that Rossi engines are set up such that the chart you quote works for Rossi engines. If we all ran Rossi's that would be fine. But Rossi is a fairly odd-bird, not what I would consider typical of your average sport engine in the .25-.61 range. I don't know if they still are, but if I recall Rossi was one of those Eurpoean engines that was tuned for 0% nitro and could become quite the firebreathing monster when sone nitro was added. Very high compression ratios. I could be mis-remembering, and it's been awhile, I haven't seen a Rossi in person in years.