High level estimates

I am currently working on creating High level estimates for a project that we are working on this year. I came across this method called PERT - Program Evaluation and Review Technique This technique allows you to plan and schedule high level tasks to complete a project, and visualize those tasks on a PERT network chart that shows time estimates between tasks and allows for task to be moved around, added and deleted as requirements are refined. I am currently researching other models for doing high level resource and time estimation of a program or project.