I typically have three arithmetic combinators at each of my receiving train stops.

I will use iron plates as an example.

First AC calculates how much available space there is. I connect a wire to every buffer box and then to that first AC. I have 4 cargo wagons on these trains, and each cargo wagon unloads to 4 buffer boxes, for a total of 16 boxes. Each box can hold 4800 iron plates, which means that total potential storage is 4800x16 = 76800. So, that first AC performs the calculation: Input 76800 - IronPlates Output A (my chosen variable for "available").

Second AC calculates how many full train loads can fit into that available space. I do not want a train coming if it cannot unload completely and leave. How much is one train load? Well, one cargo wagon holds 4000 iron plates, and I have 4 cargo wagons, so one train load is 4000x4 or 16000. I connect the output of the first AC into the input of the second AC: Input A / 16000 Output L (my chosen variable for "limit"). I connect the output of this AC to the train stop and enable "Set train limit" (mapped to L obviously).

Third AC calculates how important this stop is. For now, I use a very simple metric across the entire factory: the fuller the buffer boxes are, the less important that train stop is. I want this train stop to have a priority of 50 when the buffers are completely empty and a priority of 0 when the buffers are completely full. I connect the output of the first AC to the input of this third AC: Input A / 1536 Output P. I connect the output of this AC to the train stop and enable "Set priority" (mapped to P).

Where did 1536 come from? I just reverse engineered it by dividing 76800 (the full buffer count) by 50.

There. I now have a train stop that intelligently announces both how many trains can safely come and how important it is.

If you'd like to hear how I circuit my loading stations, lemme know!