[andrewrn]

Thanks for trying it and sorry about those errors. All fixed now.

You're very much not alone on the UX friction-- that's what most people have said. My gut says that what is fun about Fermi estimation is chaining the factors, (which OP's clone doesn't have), but it's not a trivial thing to package into an intuitive UI. So I'll have to think about it a bit more. If you've got any more ideas or suggestions, I'm all ears!