The development of COSYSMO has been an ongoing collaboration between industry, government, and academia since 2001. INCOSE provided expertise as well as a forum for collaboration between stakeholders that led to the eventual development of the model. In 2004, we provided eleven lessons learned from experiences collecting systems engineering data from six companies in collaboration with the INCOSE Measurement Working Group and the Practical Software and Systems Measurement (PSM). These lessons were focused on the development of COSYSMO that was motivated by a similar model from the software domain, COCOMO II, but was a first of its kind for systems engineering. Now that the development phase of the model is completed we take a retrospective view of lessons learned during the ongoing validation phase of the model and present new lessons learned that should help cost model developers, academic researchers, and practitioners develop and validate similar approaches. These lessons include the need for more specific counting rules, an approach to account for reuse in systems engineering, and strategies for model adoption in organizations.