You might try this. If you look at the NSW "Ratings Based Handicapping Template" a horse jumping from a Country Class 3 to a Saturday metro 3YO&UP Class 3 would expect to carry 4-6kg less in the city. In fact the template basically gives this as an example.

"A Class 3 horse rated 77 could ascertain the following weights across the template
...
Metropolitan Class 3 = 58kg
...
Country Class 3 = 63.5kg"

Just an idea, but it does throw Don's scales out a bit. I guess we have moved on 16 years since that book was published.