Unfortunately, there is no mathematical formula and no 'magic' spreadsheet that would be able to just calculate the effort based on some basic information about the source environment for the migration. But, there are some lessons learned from completed IBM Business Process Manager migrations that make it easier to come up with a reliable estimate. These lessons learned are summarized in this article.
Note: It is always strongly recommended for an IBM Business Process Manager migration project to involve experts from IBM Services. The following web site has more information: http://www.ibm.com/developerworks/websphere/services/discovermigration.html. IBM Services will run or guide the migration as a project with clearly defined phases. The initial phase (Discovery) aims at coming up with a reliable estimate and plan for the necessary migration activities.