I don't use a definite recipe of proportions when making the wash but here are the guidelines I use:
The wash is prepared by first making a stock solution of 4:1 water to PledgeŽ Floor Care Finish with FutureŽ Shine. I place a few squirts of black, reddish brown, and green (I target a color that I wish my wash to be...adding colors until I achieve the look I want) into my storage bottle. After mixing the pigments I add aliquots of the clear stock solution and mix it together until the clumps of paint are fully dissolved into the solution. I then add more of the stock solution, testing the strength of the wash on a piece of paper until I am certain that the wash flows smoothly. You can always dilute the wash by simply adding more of the stock solution to it when you want less effect.
I am not completely sold on using this on my warbands because it gives a very oily shine to the miniatures, however, for terrain pieces it is much more economical than using the small pots of GW wash for large surfaces.
Hope that helps.
Here is a link for Magic Wash which lists some other products available by country that are similar to the PledgeŽ Floor Care Finish with FutureŽ Shine.