Say I'd like to create a factory that produces each science pack at a rate of 1/s. How can I figure out the total # of iron/copper plate machines (etc, etc for other int ingredients) needed to support this production chain?
I've tried subfloors which just gives totals for one specific science pack. I've tried breaking down all the products on one floor all the way down to plates which seems super hard to get right because list order seems to matter so there's lots of separate entries for plates.
Is there an easy way to do this I'm missing?

If I'm understanding you correctly, you just want to know how many copper and iron smelters you need right? If so, then yes, you need to specify all the recipes you want to use in that chain. The mod couldn't know which recipes to use and so on otherwise. Using one subfloor for every science pack is a smart approach to this btw. If you break all the chains down, one per subfloor, the ingredient amounts at the very top will tell you how much ore you'll need.

Oh I also just realised, if you want to tally up all the smelters in one place, break down your recipes down to plates, without adding the smelting recipes on every floor. Then, on the top floor, add your smelting recipes once at the end, and they will produce enough for all the other recipes.

Hopefully this helps, feel free to ask if something is still unclear.

Got it, I'll try that out thanks.

I'm not sure what the limitation around recipes is..the only recipe for plates is ore -> plate right? I've seen other calculators handle multiple recipes (eg oil) by allowing the user to set a preferred recipe.

When I go to setup a bus I want to know the number of belts (and therefore machines) ultimately needed for basic materials like plates/steel/etc hence this request. Basically looking for info like this in-game: https://kirkmcdonald.github.io/calc.html#zip=bY1BCsMwDAR/41MFSY8BP0ZRRCtiW0ZSSvr7NmfntszA7IaBeYYJpmQYnD1Rz8+0conMZzd2hzBs3tUCLpw+4nnVM0lw9YxHaMUQbeAk3IihI+2LLfOj6Es8hEZTpUigfUdDb65CWEbTTbeD7p+OuII3Of8vHvAP

Yeah, I suspected that this is what you wanted, but it's just not something that's really doable while still supporting any and all mods. KirkMcDonald can make a ton of assumptions with their algorithms because they knows exactly what recipes it needs to handle, and there are basically never two paths for the same thing. For the very few cases where there are, there is an option. I can't do that because FP needs to handle every modded use case.

Now, what I could theoretically do is add all the recipes where there is only one choice automatically, as an option. This goes a bit against what most people use the mod for, because I suspect most people don't actually break things down all the way every time, and it also will only really work in vanilla situations. I actually didn't really think about adding this before, I wrote it down and will see about adding it maybe. But it won't be able to work exactly like KirkMcDonald, especially for oil/uranium and similar situations. Thanks for getting that thought going!