inexakt, but ungenau. Is this pure convention or are there any rules of thumb what prefix to choose when building an inversion of an adjective (for example pronunciation)?
A pretty good rule of thumb is whether the word to be negated is a Fremdwort or not. Latin words are inverted with in-, Greek words are negated with a-, while typical German words are inverted with un-.
Inexakt, indiskutabel, inakzeptabel, invariant, inoperabel
ungenau, unmöglich, unwohnlich, unbehaglich, unversehrt, unverletzt, unbeschädigt, undenkbar, unfreundlich, unnahbar, unverschlossen
As usual, this is not a 100% rule. For example "Natur" is latin as well, but natürlich is negated to unnatürlich, because of the -lich suffix, which is typical for a German word.
Also note that you should avoid the un-prefix where possbile, for stylistic reasons. Almost all un-words can be expressed by an alternative word, without changing the rest of the sentence.
- This first link is the best to answer your question, it also covers the a- negation and where in- turns to il-.
- A blog posting arguing to avoid the un-words.
- Examples for words starting with un- that are not negations or that do not have an equivalent word without the un-.