Hi, Roseann. Everybody that I know just pours in a little of this and a little of that when using paint for stain, but then they are not repetitively doing the same colors or work. Your best approach might be to get some wood and measure exactly as you test colors and percentages. And I don't know if you are using paint purchased from one manufacturer and precolored or using paint colored by a paint store. In about every case though, there is variability in color and viscosity.
Have you considered using aniline dye, either water based or alcohol based? There would be a lot less variability as long as you mixed exactly the same.