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There are two different reasons people use these products. One is for permeability the other is to get relief from lot coverage regulations. In fact, many of the people who have spec'd permeable pavers for drainage reasons only have done so to get relief from other regulatory bodies, such as a conservation commission. Often times, there is no specific goal for how much runoff is reduced is the point that I'm making.
I and others who I worked with (civil engineers) spent a great deal of time last August trying to find the information we needed to calculate infiltration. All we found from any of the manufactureres were qualitative assessments and no quantitative assessments that we could then apply to our own conditions and soils.
We were pretty well left with hiring in a geotechnical engineer to make specific calculations, or to stay with the generalization that more infiltration was better than less infiltration. Due to the size and scope of the project (part of a residential driveway ~800 SF), we chose the latter. I would expect that the same will hold true for most projects that are going to be left for a contractor to design.
How much runoff that gets absorbed is also determined by storm intensity, slope, and perc rate of the soil underlying your base preparation as well.
The bottom line is that if using permeable pavers is done to feel good about reducing runoff in a qualitative way or to reduce lot coverage, do the best you can at balancing your quality of construction with increased porosity. If it is done with specific runoff values as a goal, they should not be turning to landscape contractors to design what the manufacturer can not even outline in their technical bulletins, or what local civil engineers can not calculate. The contractor should have a set of plans and spec's to follow in the latter case.
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