Mixing Acrylic Paint With Water ( And Storing It )
Paint is a chemical with a blend of several characteristics. Two of those characteristics are:
- Carrying a pigment (color)
- Sticking to a surface (glue)

A 50/50 mix would reduce the binder (glue) in the paint to half of what it was when it came out of the tube. If you were painting on paper or a medium that absorbed the paint you might get away with 50/50. Some paint makers recommend never thinning paint more than 25% with water (Liquitex thinning recommendation - not an endorsement).


  Think of the polymer molecules (the binder) as forming a sort of glue that holds the pigment onto the surface. When this glue, known as polymer, gets thinned by water, it loses its strength to bond. When the paint pigment is left unbound, over time it will release from the surface on which it was applied (usually canvas) and begin to flake off. "Don't user Water to Thin Your Acrylics"


Miniatures don't absorb paint so you are relying on the strength of the binder to hold the paint onto the miniature when dry. it would be best to "stick" to the paint maker's recommendation on using water to dilute paint.

I'm a little paranoid. When painting I use as little water as possible for spot thinning. When I need thinner paint for a large surface I use a painting medium to thin the paint. I don't want to do the work of painting only to have it mess up months or years down the road because I used too much water.


  Acrylic painting mediums can be used to thin acrylic paint in unlimited amounts "How to Thin Acrylic Paint"


You mention creating your own little pots like the ones the Citadel workshop uses.
Adding tap water to acrylic contains microbes and minerals - this can damage the paint. If you use water and plan to store the paint it's best to use distilled water. It's safest to use a painting medium.  

The keys to storing paint are:
- Don't let it get contaminated
- Don't expose it to air.
- Use clean, air tight containers.


  Believe it or not, tap water is a contaminant. Tap water contains microbes and minerals. If you add tap water to a jar of acrylic paint and store it for a few years, it will probably develop mold.  from Does Acrylic Paint Go Bad?


The linked article above has good information on what makes paint go bad and how to store it.