How to deal with imbalanced requests for multi-good multi-wagon stations?

[mrvn]

I have stations that has an assortment of goods they provide in quantities that require trains with multiple wagons.

Lets look at a concrete example: I have iron and copper ore patches right next to each other and a single train station for LCCCC trains with 4 warehouses as buffer.

For each wagon there are a number of inserters that load missing items into the train and one filter inserter (stack size = 1) that removes excess items so the train can be loaded count perfect.

Note: I'm not reserving slots in the wagons, I want full trains.

So when LTN sends a train to pick up 1000 iron ore (for simplicity all numbers are scaled down) then the inserters will load maybe 1020 iron ore onto the trains and then take 20 iron ore back out for an even 250 iron ore per wagon. This works great. Also works with a mix of iron ore and copper ore in the train.

Now here comes the problem:

LTN just send a train wanting 999 iron ore. So again the inserters load up 1020 iron ore and bring that down to 1000. That's still too much so 1 iron ore is removed from each wagon bringing it down to 996. That isn't enough so iron ore is put back onto the train bringing it back to 1020 and the loading never completes.

How do you deal with that?

PS: This rarely happens so I've kind of ignored it but when it does happen it's annoying. I've wondered if maybe LTN should have an option to always request a multiple of the wagon count because it's such an ugly issue to deal with.