I could do the math and show you the numbers why your answer works for the situations you describe, but you are correct, it wouldn’t really matter because what you have found through experience is what works for you. That’s really the advantage of having this forum, lots of experience to draw from and use as a starting point until someone gets their own experience. I find it particularly useful that collections of people are using the same gear and talking about it, specifically the Rumble line and the Zoom processor. It really gives a lot of perspective on how that gear behaves for a variety of people attempting to do a variety of different things.
The answer to any audio engineering question anyone asks me is usually, “It depends.” What kind of music? What kind of stage? Who else is on the stage? Who needs to hear you? How are they going to hear you? Who doesn’t want to hear you? How big is the audience? What shape is the audience? How far is the stage from the audience? How far are you from other people on the stage? What is the shape of how everyone is placed on the stage? What microphones are being used where? How is stage monitoring accomplished? Are the musicians wearing in-ear monitors?
Anyone who has played in a similar situation to the one someone is getting ready for is going to be able to give way more useful and readily understandable information than I can with math. My advice would be to copy a rig from the experienced and then make your own modifications from there.
As a public service announcement I’d strongly encourage anyone experiencing high SPL levels on stage to consider the options for in-ear monitoring.
The power topic is one of the most misunderstood among anyone, including engineers. I could talk about it for hours. Often times manufacturers don’t follow a standard when measuring their gear and quoting power numbers. It used to be in pro audio that amplifiers were pushed to some reasonably acceptable THD (total harmonic distortion) for a given test signal and that was considered their average power rating and their peak power available for short transient sounds would be 6dB higher. That means that a 100W amp average power rating could put out as much as 400W, but only for a very short time. With digital amplifiers they often quote a higher power number than with analogue amplifiers because they are capable of sustaining their peak power for longer. Sooooo, many digital amplifiers will say their power rating is 400W, but that’s it, it isn’t going as much above 400W for a peak, that’s all you get ever, but unlike an analogue amp you can get it for long enough that it can be rated higher. Many manufacturers of music products get their amplifier boards from a company that came out of Bang + Olufsen, the IcePower product. Open any mini-amp and there’s probably a chance this is inside. Each manufacturer makes their product special by adding the cab sim, integrating an effect or making a tone control people like. Companies don’t always invent their own audio amplifier boards. IcePower History
Something else to look for when considering amplifier power is what impedance load you are driving. Some amplifiers will put out full power with either a 4 Ohm or 8 Ohm load. Many will advertise the 4 Ohm power rating and put out half as much with an 8 Ohm load, a 3dB performance drop.
I know this is all head spinning, which is why I say use a rig that someone likes.