Instead of a heuristic, what you seem to be groping for is what is referred to as an *ansatz* in physics. The significant difference between the two is that the ansatz is the missing step in a formulation, while a heuristic is something that just appears to work but otherwise does not lead anywhere. So while an ansatz is the simplifying premise or eureka moment in the process of devising a formulation, a heuristic is merely a placeholder.
A good example of a heuristic is the sunspot cycle. Here is a passage that I published in Mathematical Geoenergy (Wiley/2018)
> "Sunspots appear to have an 11‐year cycle, making them somewhat deterministic, yet we do not fully understand the mechanism. Thus, a heuristic is applied to the sunspot cycle describing an 11‐year cycle."
The 11-year period is a placeholder heuristic used to describe the cycle but it lacks any significance beyond that. It's possible that someone will come up with a simplifying ansatz in the theory of the electrohydrodynamics dynamo and the rationale for the period will become apparent.