ReRe's Law of Repetition and Redundancy [4] somehow does not apply to you:
A programmer can accurately estimate the schedule for only the repeated and the redundant. Yet,
A programmer's job is to automate the repeated and the redundant. Thus,
A programmer delivering to an estimated or predictable schedule is...
Not doing their job (or is redundant).
I think the explanation was made in another comment where they say "don't forget to estimate time for research & design", which explains why the *3 multiplier works for me. And yes, I feel that most of the work is redundant, there's always like network services of some sort or web pages, databases, linux servers, a humain interface, other services depending on the project like a message queue or a caching server or both, background workers and scheduled code execution... things I had been doing for ~10 years back in 2012. Data structures, workflows, they change between gigs, and each gig has its own particularity, but 90% is the redundant from a gig to another: HTML/CSS/JS/Python/SQL/Networking/Storage/Linux stuff
