I do measure both the soap and the water. I usually just have under 20L of water to have place for the little extra water that the PW ads when I activate the car soap. The car soap can be a little tricky to eye-ball when it`s a 1:1900 dilution of it down to the 1:500.
I`m usually very thorough when dilute any chemicals as the bottles is often a Kwazar spray bottle and it`s of a different width in top and bottom. But has some marks on it that makes it easy to fill the water amount and the chemical after that. Also measure in the metric system. If it`s only says for an example 1:4 or 1:10 on the dilution. It`s means over here that it is 1 part chemical of 4 parts solution and 1 part chemical of 10 parts solution. But if it says 1 part chemical to 4 parts solution it is that. And with 1:4 it`s 1 part chemical to 3 parts solution. I know it`s a mess that we see it so differently. And it don`t gets that much of a difference on how you measure it.
Some have started to say the percent of the solution with chemical in it. Mostly on chemicals diluted weaker than 1:100.
We had a forum member on a domestic forum that did an app for Android that you can put in the numbers to get the right dilutions both in the wash bucket and the foamcannon and foam gun. Especially for the foamcannon it`s very usefull at least in the beginning depending on the dilution recommended. If you only goes by the dilution in the foamcannon bottle you get different dilutions on the paint with different PW and also different foamcannons. So it gets easier when you have measured your PW and foamcannon what dilution it gets you on the highest chemical mix setting on the foamcannon. It`s difference can be between 1:15-1:25. Depending on the gpm from the PW and which size of the orifice in the foamcannon makes it to a different dilutions on the vehical. This little app gets very usefull with some prewash foams. If you have a 1L bottle on the foamcannon you usually know after some time how much of the solution in the bottle you use if haveing it on the highest chemical mix setting on the foamcannon. Then if you use 0.7L you put that into the app and also how the dilution is from the highest chemical mix setting on the foamcannon you put in. And at last you put in the wanted dilution on the paint. Clicking the count button and you get the amount of the chemical you will ad into the foamcannon bottle.
Some car soaps don`t get you so much of extra cleaning ability from it if you dilute it stronger than the recommended dilution. While some car soaps like Carpro reset gets way more stronger if you dilute it stronger than the recommended dilution. Thinking it has about what surfactans and other chemicals it has in them and how they react when diluted. So personally I follow the directions of the dilution of the car soap.