The Rational Method is well documented. StormShed3G™ attaches an additional variable, f to the well known equation.
Where
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f=Design Event multiplier
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c=Rational Coefficient
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i=Rainfall intensity (in/hr)
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a=Area in acres.
The Design Event multiplier is typically 1, hence having no effect on the original equation. However, there are instances were a multiplier is associated with different design events to scale the computed value. StormShed3G™ offers a customizable table where the Design Event Multiplier can be modified.
For multiple drainage basins, the value for ‘c’ is typically defined as: 
When accumulating flows, the common practice is to compute the time of concentration, look up the associated intensity and compute the flow. In the case of a confluence of several storm sewer lines, the practice is to evaluate each incoming storm sewer line to determine the longest time of concentration. The intensity of the confluence is then computed based on the longest time of concentration. StormShed3G™ provides an additional safeguard.
In the case where there are two storm sewer line merging, the program first evaluates each incoming line to determine the line with the longest time of concentration. It then evaluates the accumulated “c” time area ratio (ca) for each incoming sewer line to determine if the longest time of concentration also coincides with that sewer. If they are in agreement, the intensity is computed based on the longest time of concentration.
Should the two tests not agree, a lesser drainage area has the longest time of concentration. This is not necessarily bad, however, should the difference in ca be great, perhaps the sewer line “A” services 10 acres with a tc of 15 minutes and sewer line “B” which services 1 acre has a tc of 30 minutes, there may be a problem. Typically, this scenario requires engineering judgment. With StormShed3G™, a threshold can be set to automatically evaluate the ca associated with each sewer line. The program will use the longest time of concentration only if the threshold ratio times the ca of its associated drainage area is greater than that of the others. The default is 90%.