Lesson planning of teachers as a research field has received little attention in terms of modelling and measuring relevant competences. As an innovative measurement approach, we developed a standardised method for analysing written plans of demonstration lessons. Our focus is on the demand of pedagogical adaptivity, i.e., the ways in which lesson assignments fit with the cognitive level of learners so that they are guided into their zone of proximal development. This conceptualisation is operationalised by using several indicators (content analysis criteria) to reconstruct and quantify situation-specific planning perception, interpretation, and decision-making. We use the data from 172 preservice teachers in Germany with their first demonstration lesson during induction. Findings show their declarative general pedagogical knowledge of adaptivity (assessed via a standardised test) is a significant predictor for the situation-specific skill of pedagogical adaptivity in written lesson plans, and the latter effects preservice teachers' self-reported instructional practice of teaching that lesson. Findings are discussed towards their implication for the design of teacher education.