Gary -
While there is a standard that defines contrast variations in graded papers, there is no corresponding standard to define contrast variations with VC filters. Therefore, it is likely that a filter set from manufacturer X will be different from a filter set from manufacturer Y (unless, of course, both filter sets are actually manufactured and private-labeled by manufacturer Z).
Instead, what you do know is that the contrast that you get increases as the number on the filter becomes larger. There are a few instances (reported here in the APUG forum) of contrast reaching a plateau, or perhaps even decreasing as the filter number increases, but that's a characteristic of the VC paper and not of the filter itself.
So the key is to find a filter set from one manufacturer hat provides the gradation you are looking for, in the format you need (size, filter drawer or under-the-lens, etc), and at a price that you consider reasonable. And then use it to learn how various papers respond. Printint with VC filters is a bit like cooking - there is no absolute formula, but rather you have to "season to taste" by experimenting with various filters to achieve the result you want.
The one thing you don't want to do is intermix filters from different manufacturers.