I prefer to brush-mix colors in much of my work; I like the variation. When writing instructions I often provide proportions for those who prefer to use them. If you struggle with mixing colors the following methods can be used to consistently reproduce the desired hue.
Mixing small quantities
To mix colors you can measure the proportions in drops, for example, to create a 3:1 mixture squeeze three drops of the first color listed onto your palette paper then squeeze one drop of the same size of the second color and combine with your palette knife.
Mixing Large quantities
Using a ruler, apply lines at measured intervals to an address label. Make sure you have enough lines to accommodate the proportions for your mix; to create a 5:3:2 mixture, you will need ten lines with an equal space at the bottom. Adhere the label to a clean paint bottle, taking care to line up the edge of the label with the bottom of the bottle. Pour the first color listed into your paint bottle until the level reaches five parts. Add the second color until the level reaches the eighth line (three more parts), Add the third color until the level reaches the tenth line (two more parts); shake well or stir with a narrow palette knife. I have used this method to consistently mix multiple bottles of the same color for use in class situations.
If you don't have a label handy you can mark the lines directly on the bottle but, depending on the colors used they could be difficult to see.