The so-called 'Lehrerschwa' is also used in the DUDEN, though again not completely consistently. We modify in so far as we lay down that /6/ has to be used whenever /r/ would follow immediately after a vowel within a syllable. An exception to this rule is after /a/, here remains /r/.
As the counterexamples show /r/ has to be used instead of /6/ as soon as /r/ forms the onset of the next syllable ('wär' vs 'wäre', 'mehr' vs 'mehrere', 'Brauer' vs 'Brauerei').
Example: 6 Lehrer l'e:r6 i:6 Bier b'i:6 I6 Schirm S'I6m y:6 Tür t'y:6 Y6 N''urnberg n'Y6nb''E6k e:6 der d'e:6 E6 verkehrt fE6k'e:6t E:6 w''ar v'E:6 2:6 BurgerKing b'2:6g6k''IN 96 W''orter v'96t6 u:6 Uhr Q'u:6 U6 durch d'U6C o:6 vor f'o:6 O6 Information QInfO6matsj'o:n But: Bar ba:r Mark mark mehrere m'e:r@r@ w''are v'E:r@ Brauerei braU@r'aI