When you build more than 2 factories, it gets confusing to manage building one model of car in different places.So, lets say I have 2 factories, one is producing 322 cars, the other is producing 125. The current info boxes show that my global production is 447, and that my selected factory's production is 125. But I need to find out the remainder. In this case, it would be 1000-322, and 678 is the number I'm looking for, so I know to raise my production in the selected factory to 678, without having to do the math manually. It would be nice to have a box that shows this number without having to do the math myself.
There are plans to implement a "Missed sales within shipping range" on the side panel for factories in 1.19. That should give you the amount you need to produce more of to fill demand. Assuming that's what you're looking for.
Your assessment of remainders would become much more difficult when you're managing 20 factories. For starters, each branch only has a limited shipping radius. Factory may be in that radius where as others wouldn't. But another branch may have overlap of all the factories, thus screwing up a simple Numbers Needed - Numbers produced deal you suggest.
In the meantime, if you have monitor space, you can open both the branch distribution window and the factory production windows. The branch window includes both missed sales at that location and the number of vehicles produced in a shipping range. You can then adjust the production amounts based on that information.
(Eventually as you get larger, it becomes easier/less micromanagy to use the districting system. The trade off however is optimization.)
That does help, but I don't think it's quite what I'm looking for. If there's 1,000 demand within shipping range, and I have 2 factories close by eachother producing one car, I want to find the number of cars one of those factories has to produce in order to reach that demand. Factory 1 produces 500, factory 2 produces 250, but my demand is 1,000. Since math is evil, it would take a couple seconds for me to find out what the 2nd factory has to produce to reach demand. In order for my total factories to produce 1,000, each has to produce 500, but one is only producing 250, and let's say I don't know that I need to bump production of factory 2 to 500, so I'd have to do 1,000 (demand) - 500 (all other factory total) to find what factory 2 must produce.
What your proposing is no different than what I am, except for the fact I'm not going to calculate how many vehicles at other factories supply. I will just be showing missed sales for the selected factory based on the shipping range.
This is because branches may not overlap all your factories, thus giving you misleading information. Because the Factory A might need 1000 units, Factory B might need 1250 units, but only 400 of those units are overlapping demand. You might end up pulling all the required vehicles from Factory B, and not Factory A because Factory B is closer to the branches and you have not set district locks. So then your Factory B isn't producing enough vehicles to fill demand that does not overlap with Factory A. Where as Factory A is now overproducing vehicles because Factory B is supplying all the vehicles due to proximity and user settings.
Now raise this to the power of 273 (the number of cities in the game) such a system would not scale up well. The information would be bad for many of the factories.
It is simpler, and more correct, to just display the number of vehicles needed to be produced within a factory's shipping radius. And let the user come up with how best to manage his production. Yes, you can still run into the above issue where one factory may require X amount and another requires Y. But if you're getting to the point of over complexity, it's probably time to switch to districting system, use auto-production system, or build your models only in a single factory (very common in real life, if you need multiple factories, spread them out)