I've found Auchterloney was actually spelled: 05/09/1775 OUCHTERLOWNIE DAVID JANET JOHNSTONE/ M Dundee DUNDEE CITY/ANGUS 282/ 0120 0265 marriage
These are Ann's parents and Tom's maternal grand parents. David was a wealthy merchant as was Thomas' father Thomas Powrie, likely how Ann met him.
The only Thomas Powrie in Dundee in the Business Diectory
Name: Thomas Powrie Dates: 1801-1825 Location: Town: Dundee County/Colonial: Scotland dundee Occupation: Occupation: merchant Gender: Male Address: Address: Wellgate Town: Dundee County/Colonial: Scotland Source Date: 1822 Source Info: Listed in, [Dundee, 1822] Dundee Delineated; or, A History and Description of that Town, its Institutions, Manufactures and Commerce
That would make his paternal grand parents John Powrie and Margaret Thomson of Reswallie.