Don't try! Estimates are dangerous and should be avoided. When someone needs a number from you tell them they can make up their own. It doesn't matter. Better show that you can continuously deliver something of value, say each day. Let the middle managers or team figure out what that means for the bigger picture. I can recommend these presentations:
https://www.youtube.com/watch?v=QVBlnCTu9Ms https://www.youtube.com/watch?v=bicBUVbeR58