Scientific Quality
There are several approaches one can take to the question of when a study is good enough to receive a positive assessment or statement. Regarding the scientific quality of the study, one could, for instance, rely on a set of more or less objective thresholds for the overall uncertainties associated with the study's conclusions. This approach is familiar in the context of power calculations and sensitivity analyses. Similarly, one can consider the overall uncertainties of a research project's results and from there assess whether the experiment is scientifically good enough to proceed.
One could also consider the existing knowledge in the field. If there is relatively little knowledge already available in the field, it could argue that a trial is scientifically good enough to proceed, even if it can only contribute with relatively sparse or somewhat uncertain knowledge. This approach is more flexible than the aforementioned but simultaneously risks setting the bar too low.
Ethical Quality
When it comes to the ethical quality of a study, it's beneficial to consider the aforementioned distinction between what is ethically permitted, required, prohibited, and supererogatory. In the good enough trial, one stays within the bounds of what is ethically allowed, actively does what is required, and refrains from what is ethically forbidden. However, one does not necessarily do more than what duty demands. Unlike the outstanding trial, the good enough trial does not go beyond the minimum requirements from an ethical perspective.