Here, by the looks of it, no RATE calculations are required.

Just VOLUME...and only for the 2-year 24hr design storm to boot, meaning:

1) Get the total volume ("Depth") of the 2-year 24hr design storm for your area.
FOR BOTH PRE AND POST DEVELOPED SITE:
2) Get the respective site areas
3) and their run-off coefficients...these are usually quite standard (http://www.lmnoeng.com/Hydrology/rational.htm):
4) Calculate total VOLUME for both pre- and post developed cases "generated" by each area...V = run-off_coeff x Area x "Depth"
5) Sum the Total Volume "generated" by pre-developed case area and compare to the Sum of the Volumes of the post-developed area.
6) You need a 25% improvement.

The sample "Sample Calculations – Cheyenne Botanic Gardens" in the Documentation Tool Kit is a perfect example.

Note: the "Depth" of the 2-year 24hr Design Storm is a statistical value deturmined from Depth-Duration-Frequency (DDF) Charts. These charts show the relationship between the rainfall depth (volume) and the corrisponding rainfall duration (minutes). This data is required for 1 and 2 year statistical return periods.

(Hi Gregory, you may want to move the following info somewhere more suitable)

1 and 2-year 24hr Design Storm Depth and Rate values should be obtainable from your local meteorological office. At least the data required by statisticians for methods as described by Ernest W Tollner (see below), should be obtainable here.

Further reading:
Natural resources engineering By Ernest W. Tollner

Theoretical Background Summary for German Speakers:
http://www.boku.ac.at/iwhw/hydsem1/folien/hydsem2_08.pdf

Further reading on calculations relating to RATE and IDF charts:
http://europeicid2009.org/media/trudy/Sbjeschni_Schmidt_Germany.pdf