Fundemental misunderstanding of how an Arithmetic combinator works?

[vocal totem]

I'm trying to set up a remote base resupply train but I'm stumbling on getting the output signal to show what is missing. In the screenshot I'm running a constant comb of what I want, and I threw some turrets in the train to see how they affect the signal through the arithmetic comb. I've set the arithmetic comb to "each" and "minus", but that just outputs the total of the train contents and the constant comb. The wiki says that the signals are summed before the arithmetic actually happens, but if that's the case how do you perform any sort of arithmetic on the signals in the first place? I expected either the green signal would be subtracted from the red signal or vice-versa, but they just seem to be added regardless of whether I pick "minus" or "addition" for the symbol

Literally two more minutes of fiddling with it and I figured it out lol

[half compass]

it be like that

[half compass]

so for a simple request thing
you want to have it be * [-1] for the current storage
then you take the constant combinator and put it on the output of that line and all positives are what you need

and the error in the screenshot is you haev no other variable
youre doing x* [nothing]=output

I know you probably figured that part out, but I figured it was worth a mention

[vocal totem]

Yea, would be helpful for someone else if they come across this post

[half compass]

any other questions you have about combinators fren