I've never used the MM Acryl flat base, but Tamiya's works great, and you can adjust from semi-gloss to dead flat to OMFG it's chalk!
This chart is from Swanny's excellent article on Future:
Future does produce a glossy finish so many modelers will introduce a
flattening material or only use it as a pre-decal sealer. If you wish to
use flattened Future as a final coat here are a couple of suggested
ratios of Tamiya flat base for different effects:
1 part flat base to 3 parts Future = very flat
1 part flat base to 10 parts Future = flat
1 part flat base to 15 parts Future = satin
If you get too much of anybody's flat base on the surface of your
model you run the risk of making it all go white. No thinning is
required just make sure it is well mixed.