NXT is costly than Sonax. Plus both are mild shampoos which dont ruin the paint or clear coat. Oh i think you got me wrong. As i already stated earlier, i follow the NXT recommendations overleaf. So as per it, 4-5 cups/bucket of water. So instead of 4 cups entire NXT, i just use 3 NXT and 1 Sonax. That is all. Hence i roughly mentioned 60:40 ratio. I follow the same thing what you said, just add 1 or some times few drops of Sonax to the 2nd bucket to have an affective washing off dirt on mitts.