Exactly; you can only provide a more accurate estimation if you know all the details and the exact implementation and how fast you can type, and even there you have to take at least a 30% margin either way.

You can only be 100% accurate when it's done. Here's a chart: https://cdn-images-1.medium.com/max/1600/1*hS2GObQarMYhamPeV.... Basically, the closer you are to done, the more accurate your estimation will be.