In recent years teachers' education programs encourage preservice teachers to practice a variety of assessment methods to prepare them to be highly qualified practitioners who are capable in enhancing students' scientific understanding. Formative assessment (FA) -- also known as assessment for learning -- involves the process of seeking and interpreting evidence about students' ideas and actions to enhance and guide the learning process. An inquiry-based chemistry laboratory was chosen as the context of this research in which 13 preservice teachers studied the practice and application of FA. The preservice teachers evaluated students' lab reports using two components of assessment -- rubric-based scoring and providing students with feedback comments. Our goal was to understand whether guidance provided through the teacher education program affected preservice teachers' FA enactment, which was reflected in their score variation and the quality of the written feedback comments provided to students. The study findings show that the total lab report score variation decreased in the 2nd assessment due to the explicit guidance. That is, the guidance provided the preservice teachers the opportunity to examine, discuss, and improve their own assessment knowledge and scoring process. However, the rubric dimensions that were perceived by preservice teachers as more open for discussion and interpretation -- such as evidence-generating analysis and formulating conclusions -- were challenging to assess and the explicit guidance created different thinking directions that led to increased variation scores. In these dimensions the guidance exposed the preservice teachers to the complexity of a rubric-based scoring in a FA manner. We recommend that the guidance preservice teachers receive regarding FA of inquiry-based lab reports, should include aspects of how to notice and interpret students' ideas and only then respond with a formative feedback. The results of our study expand the theoretical knowledge regarding FA and have important implications for the preparation of future chemistry teachers and for the professional development of those already teaching chemistry in a classroom environment.