It's really a matter of trial and error, nearly impossible to give you an accurate percentage because it depends on your airbrush, pressure and atmosphere. Humidity and room temperature has just as much to do with it as the other factors and I find myself adjusting over the course of a session. Even an older bottle will be different than a freshly cracked one.

A good starting guideline is milk like consistency as stated above and go from there.

One thing to add, I've used iso alcohol as a thinner for Tamiya acrylic (they aren't water based acrylics by the way, the use ethers) for about 10 years with no issues whatsoever. Both thinning and cleaning. Some use Windex too, but I only use that with Future. There's no need to pay through the nose for Tammy thinner when you can get a quart of isopropyl for a few bucks.