I have been confused about the "based on driver swing speed" wording that Jeff has put in there. At first I thought he was saying that an iron DSFI of say 80 meant it was right for someone with an 80 driver speed. But after looking at shafts I was familiar with, I realized that can't be what he meant.
What I think I determined from a closer reading of it, is that it is still an iron swing speed number, but that instead of having two separate formulas, he decided to use the driver formula and then de-rate it by a percentage equal to the typical percentage of 5 iron speed divided by driver speed. So if his chart shows that a golfer with driver speed of 100 should have an iron speed of 80, then if his numbers come out to 100 for an iron shaft calculated with the driver shaft formula, it gets a DSFI of .80 x 100, or 80.
If anyone knows this understanding to be wrong, I'd love to hear about it.