I didn't know this difference between the Austrian and German pronunciation. But in German we have often pronounce the r as a "vowel", like in the pronoun Er, or in the words mehr, sehr etc.

I think the rule only applies for two identical consonants (i.e., a doubled consonant). That's the sign the vowel before should be short. Some clusters count as double consants (ck = kk, etc.). The other rule is that vowels marked as long by an "h" or by doubling are always long. Diphtongs are also always long. A single vowel in front of a non-doubled consonant may be either short or long; I don't know a rule for that. Additionally, as Rodrigo said, an "r" is sometimes pronounced like a weak semi-vowel, especially in the North.