Good question Robster - I had actually just copied the wash 'recipe' from an old email I had to myself which was incorrect. I edited my previous post to more accurately reflect what I end up with after eyeballing it. It would probably be nice to have some exact measurements, but it does vary with how thick the paint is and how dark I want the wash to be. I'm not sure I'd want to paint half a wall with one batch and then the other half with a new batch of wash since they may not match exactly. However, as long as you can mix enough to finish an entire building you should be able to match the results from piece to piece without it being obvious that they don't match.

I add the water+flow aid to the paint and make sure it flows well before adding the Future. That helps avoid the mix from getting small chunks of paint that don't mix well. The small plastic cups that I use have lids so they can be shaken vigorously to make sure everything is mixed together well.

To know if it is the 'right' mix, I start mixing the thinned paint and Future together and then brush the liquid up against the side of the clear plastic cup I'm mixing in. If most of it quickly runs back down into the cup leaving only a semi-transparent film of walnut left then it is about right. If the mixture sticks to the side and is fairly dark, I'll add more Future. To a point, it is better to error on the side of too thin than it is too dark. It is a lot easier to add another wash layer than it is to take one off.

Just a note when using the Matte Medium - the mixture will get a bit milky white making it difficult to tell how transparent it is if you add it early on. I usually mix the water/future/walnut together before adding the Matte agent so it is easier to tell when the ratio is right.

Let me know if I've left any details out or if anything just doesn't make sense and I'll try to clear it up.