Did the manufacturer of your aircraft tell you that they had made the POH vague for that reason or is it an assumption on your part.
If they had to put in everything, you would have a POH that you couldn't carry due to the weight limitations of your aircraft.
Yes, I do agree times have changed, instrumentation has advanced but you can't just make stuff up to suit what you believe without approval from either the authority or the manufacturer.
Just cos it's not in there doesn't mean all bets are off.
OK, I'll fess up.
It's a homebuilt, so I'm listed as the manufacturer, and I wrote the POH based on some existing examples. It's still an approved document though, and the inspector checked it as part of the C of A issue.
I take your point about "making stuff up", and I agree that they can't put everything in, but that may just illustrate my position.
When we fly it, we can do a max rate climb, cruise-climb, fly for range, fly for speed, fly for endurance, do a glide descent, cruise descent, max-rate descent or something else that happens to work on the day. The POH, even from a factory manufacturer, can't hope to cover everything that might occur during a flight. It includes limitations and prohibitions, which we observe, and some helpful guidance. The rest is left to pilot judgement.
Without a specific limitation, and while remaining within the designer's published operating envelope, we do what's safe and sensible. Thankfully we are still regarded as having a little more commonsense than the typical Winnebago owner!