When I started in this endevour I was intrigued by this "-1" size thing. For me the main issue was how to determine the lower size.
There is on TireRack a small helper for this exercise: http://www.tirerack.com/tires/sizes/index.jsp
If your current wheel size is 19 inches (i.e. 285/35-19), select 18 inches in the drop down box.
Locate the tire size closest to the original tire size, in this case it is 285 in width. The tire that is just one size smaller, in the width sens for 285, in the list is 275.
Now using a tire size calculator find the ratio for a tire that is 275mm in width and 18 inches in diameter that will give you the same circumference (revolutions per mile is also fine).
The rule is that you have to be within 3% for it be acceptable; in reality you should find a tire that is within 1%.
In some cases you may get tires that are not frequently used and therefore the choice is reduced. In this case reduce the width of the tire and start again.
For example the Peugeot 308SW tires are 225/45-17. In theory, the snow tires for this size should be 215/55-16 because 215 is one size from 225, as is 16 from 17.
However this size is exotic and it is not easy to get tires in this size; as a result the snow tire that is really used for this car is 205/55-16. In this size there are plenty of products available on the market - not to mention that this tire size is closer to the original tire.
So from what I see, the minus one rule is applied with respect to the radius of the rim (people take one size smaller) but is more loosely followed for the width of the tire.