Trial and error.
Not a good answer, mind you, just an obvious one.
However, such an opaque interface will cause serious problems.
type of stone will have different densities. Every
type of stone may have different compressive yields.
How am I supposed to remember that Limestone walls can hold 7 other limestone walls, but only 5 of gneiss, or 6 of granite? The number of possible permutations is extreme.
The advantage of the "needs a support every 6 tiles" concept (with possible alternate rules for soil types) is that you can just figure out how much each stone is going to be able to carry by looking at it. You can plan ahead by simply having a pre-set formula for how much of an angle you can build at.
You could make that giant dining hall with more than six tiles width if you just made a sloped roof. So long as you only go 3 over for every 1 up, you don't trigger the cave-in rules.
That's still fairly simple, and fun because it makes you have to figure out how to build certain things without undermining what you've already built, but it doesn't require you to do material science testing on every stone in the game
It be historically accurate...
An art historian once told me that during the middle ages, when the cities were growing more prosperous and everyone wanted one of those fancy Gothic cathedrals, their understanding of engineering was actually much poorer than the final product would suggest. she told me there's a lot of records of these buildings being almost done before the construction cracked and everything collapsed. Those buttresses you mentioned were designed out of trial and error.
Another interesting thing she told me was that because everyone wanted these buildings to be made, there were these traveling bands of masons and architects going around and offering their services. And my brain is relating this to bauhaus somehow, I think it's because bauhaus took their teaching system from these bands?
Maybe having a good architect in your fort would show estimates of the qualities of the stone as you select it for construction.
Yes, and part of why those stonemasons were so vital was because it literally took multiple lifetimes of trial-and-error, and they still
had many of their cathedrals collapsing on them. They were so secretive about their training and could demand such prices because nobody could replicate the knowledge they had without hundreds of years of trial-and-error.
... And now, we're going to tell players to spend years trying to learn materials science on game objects?
That's simply not going to happen. What's going to actually happen is that some small group of people are going to reverse-engineer whatever formula Toady uses, and post knowledge to the wiki, and it will basically mean that reading the wiki is absolutely mandatory reading for every single mine you dig or wall you construct.
The players revolt already at the notion of having to clean up rubble, and it's seriously hard to get people to get on board with many of the farming proposals like NPK, even when you can show how the interface can be made easy and clear for them, and even automate the process. Making players fly blind and just occasionally collapsing their whole forts on them? They would be screaming bloody murder.