The controlling technical reference for per diem amounts in the Schengen regime are found at Reference amounts for the crossing of the external borders. This is the official site for the Schengen Borders Code.
Examining the site, you can see that the reference amounts are delegated to the member states and covered by national law. This accounts for the variations in requirements.
For your question, there is no entry for Austria. This means they do not have a fixed amount and they will treat each application separately.
Taking this deeper there's the ANNEX 18 of the Visa Code Handbook which contains this entry for Austria...
Pursuant to Article 41 (2) of the Aliens Act, aliens shall be refused
entry at the border if they have no place of residence in Austria and
do not have sufficient means of subsistence to meet the costs of their
stay and return.
However, there are no reference amounts for the
above. Decisions are made on a case-bycase basis depending on the
purpose, type and duration of the stay. Cash and – depending on
individual circumstances – traveller's cheques, credit cards, bank
guarantees or letters of guarantee from solvent persons living in
Austria may also be accepted as a proof.
This is the only 'official' guideline available to you and everything else will be an informal estimate and not part of the regulation. For an informal estimate I can throw out a ballpark figure of EUR 55 - EUR 60 per day which should be sufficient (if not on the high side). The actual, i.e., 'practical amount' is subject to which location you are staying in. It will also be affected by seasonal variations.