[newfoundglory]

> If you got half of the work you estimated done by that time, I'd know you were about halfway there

And only 90% of the work remaining!

It sounds like you are imagining an estimate as a standalone number that isn't revisited or given more detail. If that's how you do it then of course you can't learn from it. It's supposed to be an ongoing process - if you said '3-6 months' when we started then after 2 months I'd expect a different answer. I'd also expect the initial estimate to be accompanied by an explanation that talks about the work to be done and which areas are causing uncertainty.