Vallejo recommend 10 to 15% thinning with either distilled water or their thinner (their thinner is GREAT) but either way estimate the drops you put in the paint cup and then divide by 10... so 20 drops paint to 2 drops water, or 3 drops if not thin enough.
What type of airbrush are you using? It's better to err on the side of 15% thinner than not enough...