"He is very precise in his job."
Translation:Il est très précis dans son travail.
I would guess that "emploi" is more directly equivalent to "employment," so the most normal word would be one that gives the sense of the actual work rather than the position, if that makes any sense.
Apparently, emploi is used more in regards to the aquisition of a job or a search for a job. Can't be certain, though.
Is there a clear rule about when to use "dans", or "a" or "en"? With my lack of experience "a" and "en" both sounded OK to me but I'd rather understand about the difference unless its more idiomatic and can't be stated??
No clear rule, only more things to learn! It is very rare that a preposition can be changed without at least a change in meaning.
"Il est très précis à son travail" would mean something slightly different: it would mean that when he is at his office, everything he does (including arriving on time in the morning) is done precisely.
So, with "in his job" you are stuck with "précis dans son emploi/boulot", which is a technical aspect (as a watch-maker, for instance).
Why "minutieux" is considered wrong?
I knew that "minutieux" and "méticuleux" can both be considered correct.
A number of more or less close synonyms can be found for "precise". Duolingo teaches you "précis" because it it the direct translation. In real life, you can hear other adjectives, like minutieux, méticuleux or rigoureux to qualify someone "precise". But Duolingo cannot list all of them.
When i say travail, it says boulot is correct. When i say boulot, it says travail is correct!
duo lingo just claimed that "job" is correct. This is the first time I've been told that. I always thought "job" is English.