I'm glad you posted this, the acceptable maximum pressure-rpm settings are very much specific to each engine/aircraft. I hear this "keep it under squared advice" a lot from instructors just blindly repeating and without specifying that the advice is specific to only that training airplane (If they have even actually read the whole poh!). It works as a training failsafe setting for a student to fallback on when they need to manage mental workload, but otherwise not accurate information to teach.

From what i can see, people get confused between vacuum and manifold pressure. First, lets get some terms set. Vacuum is a relative term. Same as cold. there is no such thing as cold. Its just less heat. So, a perfect vacuum is 0 absolute pressure. If absolute pressure starts at 0, it can only go up from there. That is what your manifold pressure gauge is reading. Pressure above a perfect vacuum. Most have heard of 14.7 psi as standard air pressure. That number is the weight of the air in a 1 inch column from sea level all the way to space. 14.7 psi is also 29.92 hg. So your pressure gauge is reading in hg, not psi, and starting at 0 hg, or 0 psi. When the engine is idling, the throttle plate is closed, and no load on the engine, vacuum is high, manifold pressure is low. When you open the throttle plate, vacuum goes low, and pressure goes high. Which means since you have less vacuum, you have more pressure. So, get the term vacuum out of your head, and just think about pressure above 0 absolute pressure, and the manifold pressure gauge starts making sense. The idling engine has the lowest manifold pressure, and the full load engine has the highest manifold pressure.