The key for me is to be methodical and make notes of everything.  Example:  I recently needed to match "Arctic Orange" for the vertical stabilizer on a Cold War era F-102.   I didn't find anything that quite matched photos (another source of error and confusion, but it's what we have).   I generally work with MRP paints, so started by blending International Orange and Insignia Red.  These first stages can be pretty rough approximations.  Start with 1:1.   Maybe 5 drops of each to get enough to cover a plastic spoon for a sample.  Not there?  Try 7/3 or 3/7.  That gets you headed in the right direction.  Let's say you like how 7/3 looks.   Fine tuning time.   Make a 30 drop batch, 21/9.   Still happy with it?  Great!   If not, fine tune it a bit more.   With the larger batch, individual drop size becomes less of a variable and the increments get finer.  20/10 or 22/8.   When you are satisfied, make a batch.   I generally do 30ml in an empty Tamiya bottle using whatever ratio I was happy with.   Label it with your paint numbers and ratios and write it down in several places.   I spray a sample card, and also put notes in a spreadsheet of materials used in completed models.  

OK.   Way too much OCD there for some, but it's how I like to do it.     When you need more in a few years for that F-106 you decide to build, the process will be simple unless the manufacturer discontinues or drastically changes the formulas.