At the scale of 1400 stories, using Little's Law as an estimation tool would become tractable. As you proceed through the stories you would have an average late

At the scale of 1400 stories, using Little's Law as an estimation tool would become tractable. As you proceed through the stories you would have an average latency and throughput. When multiplied these give the total of all stories either in the backlog or in-flight. Then you solve for when that number reaches zero and you have a fairly decent estimate of the 50% completion time. What you couldn't do well is say when particular stories would be completed, or what the 80%/90% etc times would be. You'd need to use something more sophisticated.