You are just throwing more complicated things in there that you just don't need.
The basic logic you need already exists in your VI. You just need to add another copy of it. IF the boolean switch is TRUE, then SUBTRACT the amount you want to decrement. If FALSE, then subtract 0.
Make the boolean switch a Latched When Released button (so one click gets registered and it springs back to False).
The additional subtraction is just one additional operation on the wire you already have that holds the value for the amount of water in the tank.
Very good. It looks like you implemented the new feature similar to how I described. it seems like it is behaving like you said you wanted to.
Thanks to you I have learned a lot! Sincere thanks! In order to show you, that your time is not wasted I attache my program, with some modifications, where I used the tips you gave me.
Don't be confused about names, I use my native language.
Thanks once again!