To advance impact assessment (IA) practice worldwide the International Association for Impact Assessment (IAIA) has long promoted and published a series of international best practice principles, including the recently revised best practice principles for IA follow-up. IA follow-up refers to any kind of undertaking that seeks to ‘understand the outcomes of projects or plans’ that have been subject to IA. To support the implementation of these principles worldwide, a global guidance document has been developed (published by IAIA in 2024). The aims of this paper are to report on the approach undertaken to this guidance, applying a Delphi method, and to reflect on the utility and learnings derived from the process. To this end, the method of reflexivity was utilised as well as consideration of the broader literature. Overall, applying the Delphi approach in combination with international workshops helped calibrate international guidance that will be meaningful to a broad audience and relevant for unlocking worldwide experience. A key learning was that establishing and communicating international guidance generates tension between detailed explanations relevant to specific contexts versus generalization and overview.