Unloading Cargo onto planet: Who to vary drop quantities

[Bowden Russell]

List,
In a game I am playing, I have just captured a planet that has a cargo capacity of 600Kt. Okay, on my transport ship I have 160 troops and Weapons platforms.

The problem is that I don't know how to tell the transport to put down 30 troops (mass: 300 kts) and one or two weapons platforms. If I ask it to put troops down it puts 60 down, no less, and therefore I can't add any weapons platforms!

How do I vary the amount of cargo to be unloaded?

Thanks,

Bowden