In real applications, there may be a number of other considerations.  I added a attachment that is good on this method, but I do not think this is a necessarily the preferred method as suggested in the paper.  But assuming this is like a class problem, do calculations with best information you have or can estimate relative to storm conditions on each watershed (each should be less than 300 acres if you are consistent with assumptions).  Then similar to unit hydrograph approach, develop a composite hydrograph by adding the the three based on time to peak and time of recession calculations to culvert site.  The time of recession is typically longer than time to peak. The approach is a good training tool to help visualize many of the factors that can enter into a decision.    

If you have any stream gauging stations on small streams with peak flow and storm data in your area of interest, you may  method to help validate its usefulness.  

To help confirm applicability, you may find sites in similar physiographic area that the culvert has remained in place for many years.  Run the rational method on that situation and determine results for comparison.  Another approximation if you know how to read stream bankfull indicators, is to take 2 times the bankfull depth at thalweg and determine the stream cross section at that elevation.  Use that cross section for sizing the pipe, and compare with any other methods.

https://pubs.usgs.gov/sir/2005/5116/pdf/sir2005-5116_3.pdf

Application of the rational method is based on a simple formula that relates runoff-

producing potential of the watershed, the average intensity of rainfall for a particular

length of time (the time of concentration), and the watershed drainage area. The

formula is

Q = CuCiA,

In the calculation of the concentration time it has to be included from the farthest point from the basins to the location of tube 3.