I've been using the checkbox but I wish it didn't immediately launch the rocket when that number is hit. I want it to keep filling as long as the items are in the logistic network, launching when the rocket is really full if possible and only launching at the minimum if the logistics network is out.

Edit: I think the answer might be to have a dedicated silo for that item and load it with inserters. Will it get launched automatically when the platform arrives as long as more than the minimum is loaded? I'll have to give it a try next time I have a chance.

Edit 2: Yup. That's it. Make a dedicated rocket silo, don't turn on automatic requests, fill it with the items using inserters while the platform is en route. When it gets there, the rocket will automatically launch with whatever you've put in there as long as it exceeds the minimum that you set. By the way, don't try loading two different items into the rocket. You can only manually launch those.