Unfortunately there is no defined calculation to determine the CE value. For projects that are not using any controllers, you would simply input a 1. But for projects that are including controllers, you will either need to have manufacturer data proving this or have a landscape architect run calculations. However, what those calculations include is not defined and up to the discretion of the landscape architect and the USGBC reviewer. Also, I’ve yet to see a manufacturer that has a defined number. In other words, it is generally a difficult number to prove but worth a try! Good luck and let us know if you find a calculation that was accepted.
You also might want to check out the LEEDuser Strategy page on "Filling out the LEED Online credit form for WEc1" page: