Easy, peesey, lemon squeezy! Go to a pharmacy or surgical supply store and buy a sleeve of disposable plastic graduated medication cups, the kind used to administer liquid medications. Most of these have graduation scales on both sides, one in milliliters and one in ounces. Either or, you can use the scales to mix any amount of paint in any ratio you choose. I have been using them for years and because I am an RN, I get them for free. A sleeve of 100 cups usually sells for no more than ten dollars. I believe they can be found in Office Max also. If all else fails, you can purchase them online too.

I use disposable plastic pipettes ,you can get them of ebay for not much money ,as for mixing paint I just guess at roughly 50/50 and it seems to work O.K.
N.

You can buy all of the measuring devices you want but paint varies between brands, and even between colors within a particular brand. It all depends on what you want to do.

I have a top-load airbrush. I add the paint I think I'll need, then some thinner (reducer), hold a finger over the nozzle so that air back-flows into the paint cup. That's how I mix thinner and paint -- on the fly, eyball-style.

When it looks to be the consistency of 2% milk, I test it out on a sheet of 0.040 sheet styrene. If it looks good, then I continue with the painting.

If not, I adjust with more thinner and or less/more air pressure.

If you want a thin line, less pressure with very thin paint.

If you want coverage, less thinner with MAYBE some more air pressure, but more air means more overspray. You just have to experiment.