I primarily use a mixture of 1 part Winsor & Newton Flow Improver : 1 part Liquitex Slo-Dri : 2 parts distilled H2O for any of my paint brands.
I think I got that formula from an article by Jen Haley years ago, so its a good starting point, and it works well for the most part. Don't ever be afraid to experiment though, there are lots of different brands, and every artist needs to use what they like... for that matter, I *still* experiment when I have time, as I have lots of Retarders and Flow Improvers left over from Reaper, Golden, Vallejo, Liquitex, etc. (I'm a paint geek... sue me )
Now, when I'm doing armies, I tend to use future with water (about 1:4 future:h2o) because its waaaay cheaper, but individual/showpiece models get always get the "good stuff."