If you mix up a little of your paint, stick your mixing stick (since it's a small amount a popsicle stick works good) into the paint and pull it out. If you were trying for a solid coat, you would count the drops off the stir stick and when you get to say 9 or 10 drops before it quits, you can count it good. Since you're spraying with an airbrush, the paint needs to be a bit thinner. If you add some more reducer down to where you get 3 or 4 drips, you will probably be pretty close considering you are trying to give your model a weathered look and not an out right solid color coat.

Regardless, different colors can have a different viscosity right out of the can so once you get it close, check it out on a piece of scrap the first time to check and see if you're getting the effect you're looking for.

Good luck on your new career!

Paul