1. Each player has a number of 'flagged' interactions, automatically calculated based on a hash of their character and their cooking skill. These interactions must be at least x:xx in positive value (suggestion is 5:00). Each interaction flagged as such gives a dish cooked by that character a chance to become a masterpiece.
2. Each player also has a number of flagged interactions, always negative, which lower that percentage greatly.
3. When a dish is cooked, the interactions are flagged, tallied, and the percentage used to determine whether the dish is a masterpiece or not. This percentage may not go above 95%.
When eating a masterpiece, the positive interactions and ratios are all that matter in the line of whether or not they gain a gastronomy point from it.
Theories that support this include research done near the start of Tale, wherein players were able to discover sets of two ingredients which they could then combine with others for a viable array of masterpiece dishes. Although rare, it may be that some players are nonetheless still able to use these two-point, high-potency interactions to create cheap, effective masterpieces.