Here is a DP tutorial that includes powercopies and rules. Instead of creating a range of powercopies with a uniform variation, I model it as lumber with specific sizes. The rule then allows for specific shapes to be placed depending on the context of the framework. The rule is also dynamic so as the context changes the rule is able to update the geometry.
Here is the simple text required to make the rule. Both "depth" and "joist_length" are linked parameters.
if joist_length <= 110in
joist_length >110in and joist_length<145in
joist_length >145in and joist_length<185in
joist_length >185in and joist_length<225in