"Fill Level" slider in cargo ships assigned to trade contracts

I'm a bit behind the times on this as I only just setup my very first trade ever since I started playing the game. I'm using the Veh2 -> Iron trade with a size 8 dock. At this size, it costs 375 Veh2 and gives +5k iron. I had over 2k Veh2 in storage but I did not realize that the ship would continuously fill to it's max capacity (750) every time it docks even though the trade only costs 375. I also had a sizable amount of ore storage to act as a buffer.
It would be really nice if the cargo ships (when assigned to a contract) provided a slider to set the max fill level of the outbound item. This would help prevent inadvertently depleting a stockpile of some item.