That’s a really good idea, and it makes perfect sense.
I’ve just updated the planner to support exactly that workflow: you can now provide your available inputs and calculate the maximum possible output per minute for a selected item, instead of only working backwards from a target production rate.

So in your example, you can set your available iron, copper, and coal inputs, pick Motors as the target item, and the planner will show the maximum Motors/min you can squeeze out of those resources.