Could you elaborate on this? ...So for 1:72 scale does this mean if I need a total volume of 15 drops(just an example) of mixed/thinned paint at a ratio of two parts paint to one part thinner then I need seven drops of base paint, three drops of white paint and five drops of thinner? I assume the white paint is added to the base color before the thinner.
Well, you are on the right track. Understand first, there is much debate on this issue - some agree with this concept (as I do), others disagree. In the end, it's pretty subjective any way you shake it. That being said, it's also fairly simple, and I'll use rounder numbers for an example:
For a 100 drop volume equivalent of paint, 70 drops will be of the base color and 30 will be white for 1/72 scale. For your 15 drop example, 4.5 are white, 10.5 base color and so on. As Im sure you've noticed, this takes no account of the thinner, which adds nothing to the color itself. It is only there in the first place to make the paint flow better and snuggle down "tighter."
Unlike the Scale Color Effect, thinner ratio's are more a matter of trial and error. There are some formulas that are fairly constant, though, and I adhere normally to an 75:25 mix, paint/thinner. Some say a 50:50 mix is correct, others just wing it. But this 75:25 rule of thumb has worked well for me as a starting point. We'll see why this less-than-precise method is okay, in a minute. SO, for your 15 drop volume of color-mixed paint, you would add something like 3-4 drops of thinner to achieve a total of 18 drops.
Do all this mixing in a small jar, shot glass or whatever is suitable and then transfer the needed mix to your airbrush cup with a pipette. For larger volumes, just pour the mix into the airbrush paint siphon jar. If you're using a bristle brush, just paint out of the mixing cup.
You'll want to mix more than needed, though, because practicing with it BEFORE you spray it on the model is important. See, there are variables at work, even on the small scale we work in. ESPECIALLY on a small scale, is probably more like the truth. Humidity, temperature, air pressure, paint volume at point of impact - all of these affect full scale painting, and they have similar effects when factored into scale model spraying.
There're probably simpler ways to do all this, but I have never had much luck with dogmatic methods. I stick pretty close to the numbers mentioned, and brush painting is a bit more forgiving than airbrushing, but either way, there can and will be adjustments that you must be prepared to make. This is why there is that inexact "artsiness" to the process.
After your painting session, I suggest you resist the urge to be TOO frugal by saving the little dribs of paint you are left with. Some people put it in yet another bottle to save for "later," some just toss it back into the source bottle, figuring it aint enough to do any harm. In the end, that's false economy and isn't worth the trouble. Toss it and clean up.
I hope this helps.