So many things can go wrong. But how do you assess, where to draw a line? What could be used as a guiding light?

Currently I try to use the following scenario as orientation: If a completely new developer would walk in and wanted to install or build or contribute to a current system/project, how much I would have to explain to this person? How many, "yes, true, but only if" style statements would I need to make?

The optimum here is, that a developer will contribute, without me needing to explain or motivate anything.

Do you use a certain approach? How would you describe it?