It's not an old wives tail in normally aspirated ( non turbo and non supercharged)small motors. Running them over square lugs them.
Think of it like this. Putting your geo metro in 5th gear and running at 1000 rpm around town. It lugs it and is not good on the drive train.
Your obviously moving into a new category airplane and the rules are different. But don't forget the "Ol wives tail" when you go fly a hawk xp or Cardinal.
That's all I'm saying.
For the most part, the "Square-ness" factor for NA aircraft comes straight out of most POH's and thier recommendations for cruise power settings. Almost all makes have these cruise restrictions: Essentially <75% power. however you determine that to be. The maximum RPM and MP regardless of FF is the "TOP of the GREEN ARC" which equates to 75% power.
I also have an NA 210D and if you want to apply maximum HP, then the top of the RPM arc is 2500 and the top of the MP is 25 inches......cant go any higher.....according to Hoyle.
Wise engine operations also dictate this from a ROP/LOP point of view. cant get any higher anyway.
So, in economy cruise; reduce the RPM and run WOT at any altitude higher than one that produces 25" MAP and then reduce FF accordingly. Find the fastest speed for the FF/MP/RPM in the tables for that altitude. If you want good LOP, drop down to the settings that produce less than 65% power.
For my 210 this happens to be 10,500 msl for 20" mp and 2300 rpm. Fuel flow is about 12gph and I get 175kTAS. Higher I get lower fuel burn, but also less speed. Lower, I get more speed, but more fuel burn. Every flight is a trade-off of ground-speed, economy and the above settings.
Overall costs dictate this and Ive trimmed it down (at 5$/gal)to about 60-70$/hr (in gas alone) or 14mpg or $0.35/mi.
Top of the MP arc will NOT be the same on all aircraft, owing to each having different losses in the induction system. This will result in different FF of each and........so on.
Still, if the OWT is still alive, its because of the general recommendation of not to be running higher than 75% power in cruise.
Thats just what Ive gleaned from some time in this business... ust sayin'