Use a story split rate in the progress report?

[deviantintegral]

One of the things I like about this throughput forecaster is that you can include a range for a split rate on completed stories. For example, if you set a split rate of 2, it will assume that each completed story leads to one more story. It doesn't apply the split to stories it created from splitting.

As is, it looks like I'd have to calculate and apply a "max stories" field for each epic to approximate this. Is there another way to include a split rate?