Jabawocky
The reason why it is a one size fits all is simply because it is.
Even John Deakin points this out in his writings.
65% power is not so high as to put you above some TCM recommendations on leaning and not so low as to result in low ICP leading to ring float and poor sealing. It also gets you from A to B at a reasonable speed in a naturally aspirated twin, and this is usually of financial benefit as maintenance costs out way fuel burn costs.
At 65% you can’t hurt anything.
75 ROP moves you away from the hot CHT values of ~40 ROP and as you are not striving for all out speed at 75% power, then little reason to run the richer mixture of 100 – 125 ROP best power mixture setting. So 75 ROP is a reasonable compromise mixture setting.
RPM set to wherever the engine runs smoothest. That should be a no brainer. Grossly under square can lead to the first point and is inefficient. Grossly over square will frighten the boss and when ROP the effective timing is too advanced.
Your bite on the metric v US instrumentation values is as meaningless as the OWT that states one shouldn’t run over square. The reality is all reading this understand what is meant by under or over square, it is a handy measuring tool. Having spent some time flying behind metric instrumented 1820 radials I find it not as intuitive as the US system.
There are not too many aircraft I know of particularly in reference to Trent 972’s question that will not be perfectly happy to run with this type of power setting. Sure there is better but that was not the question.
You won’t convert me to the APS way of running an engine, I was already there 14 years ago. As you convert to your new (?) religion try not to become too rabid and one eyed. WOT LOP is a great thing in the right place with a correctly set up aircraft, instrumentation and properly educated pilot. Remove one of these elements and you’re better off back with something like the one size fits all set up.