I have subscribed to access for the online v4 Reference Guide, and I noticed a couple of mistakes in the example for this credit. I'm not sure how to provide feedback to USGBC regarding this, so I thought I'd try posting it here.
In the Examples section, the Horton's equation is given as:

Ft = fmin + (fmax – fmin) e- k t
The "-kt" term is an exponent of 'e', but that does not come out clearly with single-line text. I suggest typing it as:

Ft = fmin + (fmax – fmin) e^- k t
The "^" is standard notation for an exponent in single line text.

Also, the weighted area calculation shown soon thereafter (and on page 191) of the v4 Reference Guide .pdf document is incorrect. It reads:
Runoff site = {(Runoff roof – A roof) + (Runoff pavement – A pavement) + (Runoff pervious – A pervious)} / A site
But it should read:
Runoff site = {(Runoff roof * A roof) + (Runoff pavement * A pavement) + (Runoff pervious * A pervious)} / A site
Weighted averages should have PRODUCTs in the numerator, not DIFFERENCES (multiply, don't subtract).
I hope this helps.