Hi @hlopezvc
I can’t speak for monday.com 
but to my knowledge the output of a formula column can never be used in an automation. I think @LaurentConroux is referring to a great initiative from @rob called General Caster, see General Caster is here. Get rid of formula columns!
That said, the example you give does not need a formula output in the automation. The automation itself can be build to subtract A from B but this requires you build your own automation action, Probably easier to use the General Caster mentioned above.