[eszed]

Speaking only for myself, it helps to think of those keys as a unit: as 'the', not 't' (then) 'h' (then) 'e'. If they're a single sequence of movements that I can turn over to my unconscious brain to execute, then my accuracy goes way up. Typing speed then is about coding more and more words into that "reflex space".

I don't know, exactly, but I think I've got most of the common words and key-combinations up to about five characters into that state, and a few longer sequences as well. (I have one long and challenging password I type frequently which I cannot accurately type letter-by-letter, but do just fine when I stop thinking about letters at all.)

If I wanted to type faster (I do ~80wpm on online typing tests) I'd practice more sequences, but I can't seem to think any faster than that when I'm doing moderately complex work. Typing latency doesn't hold me back; brain latency does.

Anyway, that's how I've developed that muscle-memory that you mention.