How defined is the threshold of graveness for mortal sins? (Catholicism)

The Catholic Church has defined criteria for when a sin is a mortal sin: 1. The sin is grave. 2. It's committed with full knowledge of the sin and its gravity. 3. It's committed with deliberate and complete consent. However, many of the sins that the Church has at some point described as grave are on a spectrum of severity and must be subjectively judged. E.g., cheating at a game can be grave if it's severe enough (how severe is that?), wages have to be just or else underpaying someone counts as grave (how does one know the threshold?), hatred and envy are grave if one wishes great harm to another person (how great? does one have to wish for the harm to actually happen, or just enjoy the thought?), lying can be grave depending on details, and so on. My question is, how can one know when the threshold is met and a sin is grave? Is this enforced or defined in any regulating, normative way, or is it just left to the opinions of the clergy?