Thanks Green1, I hear what you say, car wheel nuts can be difficult to remove. However, the rear axle suggestions provided (stand on it/ use a long leaver) would likely result in an over-tightened axle nut hindering removal and even if you could loosen the nut, changing a bike wheel at the roadside is hardly an option. Though I will try and undo the correctly torqued nut with the standard tool kit shortly and advise.
Evidently over 50-years and some 400,000 CB750K examples, there have been few if any problems associated with tightening the rear axle. It seems to withstand over-tightening and under-tightening, supported by the chain adjusters. I just want to tighten it correctly and now I can.