If an hour of real work reliably eats two hours of clock, book two. You do not have to become a better estimator to end up with a schedule that works.

A clock only counts if you can see it without doing anything. Behind you, in a pocket, behind a lock screen: that is not a clock, that is an errand.

Here is a short way to hold the whole difficulty: the now in front of you is too big and the later is too small. It fits in your head at the moment you need it, which longer explanations do not.

The more you have to look up before you can begin, the further the real time drifts from your guess. Every search is also a door left open, and some of them lead somewhere else entirely.