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