When mixing paint colours, what is a good technique for ensuring consistent ratios each time a new batch needs to be created? Some Background

Back in 2014, I made a "wand" out of a wooden dowel, and some glue gun glue for the handle. The "wand" part was a single pre-made colour, but for the handle, I mixed together a few different colours to create a nice brown-ish colour. Once I was satisfied, I began to happily paint the handle, unaware that I had not mixed enough of the colour. Once I began to run out, I attempted to re-mix the colours in roughly the same ratio as best I could remember, but obviously it wasn't exact.

I ended up having to do this at least twice, as far as I remember. I managed to get the colour pretty close, almost indistinguishable, but by the third time, I simply mixed a massive amount and painted over what I had already done.

While this was not the end of the world, it dawned on me that there must be a better way than simply eyeballing amounts and relying on memory to determine ratios.

Question

How would a more experienced crafter/painter ensure that, if they needed to mix a new batch of a custom colour, they mixed colours in the proper ratios each time?
It would be helpful to know the order of magnitude of your volumes.

I paint miniatures and so milliliters are often enough. In this case my paints are in dropper bottles and I count the droplets I add to the mix. 

For bigger volumes I would use a transparent bottle and make a level mark for each colour I add. So next time the marks could be reused in the same bottle to get the same ratio.