I was intrigued by the math, so I spent some time looking at pics of maple bonsai.
Most of the trees that looked really good were close to a 1:20 (leaf size to tree height ratio)
taller tree
The ones that were closer to 1:10 looked more like shrubs
shrub-ish
Unless they were very stylized
stylized short maple
So, I suppose that if you had to reduce it to the basic math (which seems to take away some of the art-ness) I would say a ratio of between 1:10 and 1:20
- bob