I always do the mix in those juice type containers, And this is what works for me to get the right consistency: I put the paint first, and then I start to add the thinner, a little at a time, and shake very well the bottle after each time. I always know when the mix is right because all the bubbles in the mix disappear in less than 30 to 40 seconds after shaking. If it takes longer, add a little more mineral spirits. Every time that I use the paint again, I shake the bottle and check for the bubbles and if it is necessary I add more thinner.
I hope this helps.