This is also argued by Railton in the paper “Alienation, Consequentialism and the Demands of Morality.”
However I think you’re making a bit of a conflation between act-based thinking and careful calculation. We might believe in rough, quick estimates and heuristics to guide our lives, but to still have those defined by act utilitarian guidelines rather than by common sense ethics or intuition. So as an example, when thinking about whether to be kind to someone, the act utilitarian makes a quick estimate rather than following a cumbersome calculation—but that quick estimate is to be unkind to them.
This is also argued by Railton in the paper “Alienation, Consequentialism and the Demands of Morality.”
However I think you’re making a bit of a conflation between act-based thinking and careful calculation. We might believe in rough, quick estimates and heuristics to guide our lives, but to still have those defined by act utilitarian guidelines rather than by common sense ethics or intuition. So as an example, when thinking about whether to be kind to someone, the act utilitarian makes a quick estimate rather than following a cumbersome calculation—but that quick estimate is to be unkind to them.