The OpenAPC initiative (https://treemaps.intact-project.org/) was first presented at the 44th LIBER conference 2015. During the INTACT project (2015 – 2018) OpenAPC became the authoritative source for collecting and disseminating APC cost data. For the next phase, funded by the Federal Ministry of Education and Research in Germany, OpenAPC is expanding aims and scope by involving additional bibliometric data and workflows to better estimate and calculate the costs of open access publishing and the ongoing open access transition. The potential of this approach is shown with the example of analysing Springer Compact agreements. The presentation will demonstrate, to which extend existing Springer Compact agreements have contributed to the open access shares of articles within Springer journals during the last years (https://treemaps.intact-project.org/apcdata/offsetting/). With regard to the ongoing DEAL negotiations (https://www.projekt-deal.de/about-deal/) in Germany, the presentation will also introduce a simulation analysis approach, which answers the hypothetical question what would have happened, if all German corresponding/reprint authors would have published additionally open access in Springer Compact journals during the last years. The results of this analysis can provide some suggestions to improve and adapt open access transition strategies within offsetting agreements.