Thanks very much for your post, and great question.
You are correct – the regional initial loss values provided by the ARR Data Hub are for complete storms rather than bursts. The current version of RORBwin does not currently automatically account for pre-burst rainfall, and there is therefore potential for a slight inconsistency when using these regional loss values. This is more likely to be significant for relatively rare, shorter duration storms and where total flood volume is a significant issue (eg design of dams and retarding basins).
The simplest way to account for this is to manually adjust the initial loss value down by the amount of the pre-burst rainfall (which you can calculate using the pre-burst percentages provided by the Data Hub). This works well provided that the amount of the pre-burst is less than the initial loss value (which should generally be the case).
An alternative approach is to manually adjust the rainfall IFD depths in RORBwin to represent complete storms rather than bursts.
Note that we are currently considering incorporating an automated routine to incorporate pre-burst depths in RORBwin. If this proceeds, there may be a new release in the first half of 2018 which includes this functionality.
Hopefully this helps.