Mike, you'll need to mix and experiment with the Flat Base and Future, Start at around 25 %, and test it by painting onto a piece of scrap. After around 15 minutes, it should show how it will look.
If it is too glossy, add a touch more Flat Base, and so on, but always ensure it is stirred-in and mixed thoroughly, and again before use.
If too much of the FB is added, the absolutely flat result will also show a dusty white deposit - this can be rectified in the mix by adding a touch more Future, but make sure you identify this before using the mix on a model.
Sorry I can't be more specific, but it's one of those things which can only be ascertained each time the mix is formulated.