Without having a great deal of detail about your specific situation it seems your approach is a reasonable one.  Identifying and documenting the business requirements of each impacted business area will be important to ensure coverage.

Again, sign sign on projects aren't uncommon. So you may find some information on the web related to these projects.  If you investigate multiple  single sign on solutions you can use the features they claim to offer as a basis for your requirements elicitation among the business areas in your organization.