Bones,
Common sence is the rule with cowl flaps. Watch your CHT. Heat is related also to power, more power more heat. So cowl flap settings are linked to power setting. If on one side you are doing a lot of high power low level work on a hot day your CHT will say leave flaps open, if you are at low power settings in the cruise at altitude then your CHT will say close flaps. The POH is right "as required". Don't lock yourself into doing something because some expert says do this or that unless you can see a logical argument to change. Think about what is happening under the cowl what the CHT,s are telling you. Get a EDM that monitors all cylinders as how do you know what the others are doing.
I urge all pilots to understand what's happening under the cowls and how to make your engine happy. That's the best and safest and most economical way to operate. Understand what CHT your engine is happy at, set yourself a mental alarm eg happy at 350-375 ROP but when we start heading to 400 keep an eye on it, over 400 maybe crack cowls a bit to keep under 400. Now that is only an example as every engine type is different. Talk to an engineer for specific or god forbid look up the POH.
I wish EDM,s and how they relate to good operations were taught as part of the flying syllabus.
If not make it a priory and read up on stuff John Deakin, Walter Atikinson and Goerge Braly have been saying for years. First came across their management ideas 15 years ago and started running our aircraft with their management styles and get all our pilots to understand why engine management and understanding of what they are seeing in front of them is so important to a healthy happy engine.
By the way I have nothing to do with APS. But recommend their courses.
MS