Tamiya Flat Base is acrylic, not enamel. As Woodbeck said you cannot mix acrylic and enamel paint, but since both Future and Tamiya Flat Base are acrylic they will mix fine.
The higher the percentage of Flat Base the more flat the finish will be. For semi-gloss, try about 6 parts Future to 1 part Flat Base. For a flat finish try about 4 parts Future to 1 part Flat Base. 4:1 is what I normally use for a flat finish.