Mackenzie's design career started with Alwoodley where he was the Hon. Secretary, and then developed when he was asked to lay out the nearby Moortown course - where he was a Vice-president of the new club. I do not believe he charged these two clubs a fee. But after that his services came in demand more through 1911 and 1912. By 1913 he was starting to get quite busy, and here we see the first evidence of his fees, which were typically 10 guineas including expenses for a day visit and 5 guineas per day thereafter, we see this for his courses at Silloth, City of Newcastle, Darlington and Headingley. At the latter he charged them 50 pounds to superintend their work, with 25 pounds upfront and 25 upon completion.
In 1913 alone we have recorded around 12 projects he was undertaking that year, with a similar number in 1914. So he was far more than a part-timer in the period prior to WW1 and sometime prior to the war he must have made a decision to significantly wind back his medical commitments, if I had to guess I'd say around 1912.
yes we are well up to date with Cavendish thanks to visit to the Chatsworth Archives by Richard Atherton. These are the archives of the Duke of Devonshire's various estates. A very detailed record of correspondence to and from Mackenzie (and his brother Charles who built the course) about the design and construction of this course, which makes it, along with Bury and Hazlehead, one of the best documented Mac courses anywhere.
Hope this helps. Neil.