"Do all the good you can" motto

The church I attend, the Urban Abbey in downtown Omaha, NE, has the "Do all the good you can . . ." motto printed on their coffee mugs.  The initials JW follow the quote, so the mugs don't exactly claim John Wesley as the source of the motto but the implication is there.


A 1915 collection of John Wesley's letters edited by George Eayrs attributes the "Do all the good you can" motto to John Wesley.  Below is an image from the book and here's a link to the ebook version at the google books site--



It is unclear what source Eayrs is citing, if any.  Eayrs might have assumed this motto had been written by Wesley without any other evidence than commonly held assumption.  The footnote does make me wonder if the motto was associated with Kingswood School.  Anybody out there in the blogosphere have a history of the school or access to the archives of the Kingswood School?