I just got some Sonax a few weeks ago. The first time I used it I used 2 cap fulls (100 ml?) but I think that was more than enough. So last time I used about 1 1/2 cap fulls (@ 70 ml) in my 5 gallon bucket and still had plenty of suds. Was just kind of going by sight.
I just washed my car and only used 50 ml (1 cap of Sonax) and that seems to be about right for a 5 gallon bucket...
One word of caution: that measuring cap is very brittle! I had the bottle sitting on a ledge about 3/12 feet off the ground and a huge gust of wind sent the bottle to the floor. The measuring cap is now in about 6 pieces and useless~!
The wind is pissing me off today. Wanted to wax after washing but now there is freaking yellow tree pollen all over my black car! Even putting it in the garage didn't seem to help. So no waxing today!!!