Yup, I have the same exact problem too.
I did the roughly 20 parts per 1, but found out that didn\'t seem to help, so the ratio I\'m using now is closer to 10-12 parts per 1. (I don\'t know exactly because it was just \'by eye\')
It has helped with the tiding problem, although that does seem to crop up now and again. Especially with certain colors of paint.
The one thing I have noticed though is that is gives the paint a bit of a glossy sheen. (which is fixable with dullcoat at the end)
I also seem to be able to \'tell\' if the ratios seem right by taste.
I\'m a brush licker, so I can taste the flow improver (I use Winsor and Newton and it tastes AWFUL). If there is just a slight hint of the taste, I figure that ratio is about right.
Too much (and you\'ll know) requires a bit more water.