I have a question, is it possible to have an attribute where the start value changes according to the shape which the rule file is added to. A simple task could be that the attribute should always be half of the area of the shape, so on one shape with the size 2000 sqm, the start value would be 1000, on an 8000 sqm shape the start value would be 4000.
Please have a look at the geometry Functions they provide parameters of the start shape. Additionally you can always link a rule attribute to a shape attribute, have a look at CityEngine Help for: Attributes, Sources and Connections
attr a = 0.5*geometry.area