Here's a wiki paste:
And my memory says that the studies experimented with different dilution ratios. However the conclusion was that after 10% solution there is no extra benefit of any kind. You'll only use more milk. Drink that milk, instead of spraying it uselessly.
I however found some study on grapevines, and I quote an important finding from it (and I will bold that finding to point it out) - and btw they used the 10% solution:
So in other words; Do not overdose on the ratio, the main thing is to cover the whole plant with that milky water. If you can't get it fully covered, doesn't really matter if you spray them at all. So do it properly or don't do it at all type of thing.