The peak white luminance ratio as defined in the regulation should be determined in the brightest on-mode configuration which indeed can be a pre-defined setting (such as 'vivid', 'dynamic', etc.) in the set-up menu. If the end user alters any configuration - including the brightest on-mode - through adjustments of individual parameters such as brightness and this leads to increased energy consumption, this requires that warnings are displayed to, and consent is requested from, the user (cf. last paragraph in Annex II point B.2).
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.