How to mix colors when using 2 colors for coloring hair at home?

I want to use 2 hair colors...how do I measure out 1/4 from one tube and 3/4 from another tube? Its very hard to just guess. If I put them in another container and mix them together then I can't use the applicator bottle...I'd have to brush color on. They should have measuring gadgets for the tubes. Any suggestions appreciated.