I run into this with clients all the time. It's basically a two part conversation for me: 1. A quick (few paragraph) run through of where I see the complexity

I run into this with clients all the time. It's basically a two part conversation for me: 1. A quick (few paragraph) run through of where I see the complexity that led to the estimate. This often reassures people, sometimes helps us discover a disconnect on the requirements (where I imagined something more complex than they needed), and is sometimes a waste of breath. 2. When (1) doesn't change anything "I don't litigate estimates. Based on our discussion this is my best guess at when I will be able to finish this for you, not a fixed bid. If I'm done early you'll get it early."