Is it possible to get post development run off in negative?

In response to question #1: The credits are different and require a different set of calculations. Credit 6.2 is addressing water quality and the requirment is to treat the first 1-inch (in this case) of a rainstorm. The intent is that most pollution is contained in the small frequent storms, and that developing a pollution prevention strategy to treat stormwater runoff in the smaller storms will provide an adequate and cost effective level of protection to receiving steams and lakes to minimize the effect of sediment. The requirement is actually to treat all the stormwater from 90% of the annual runoff volume. This is a cumbersome calculation so LEED has provided a guideline based on conditions in the watershed (humid, semi-arid, and arid) to help make the calculations easier. Credit 6.1 has been developed for a different reason and is more concerned with flood control and downstream flooding. For this reason, a two year-24 hour storm (which can have a rainfall depth greater than the 1-inch suggested for evaluation of credi 6.2) is the chosen basis for design, and, depending on the location, is intended to reduce the peak flows leaving a site that can cause downstream soil erosion, sediment transport, and a negative impact on drainage infrastructure. Depending on your solution, usually detention and infiltration for volume reduction for 6.1 or 6.2, the solution may serve for both credits, but the requirements for both credits must be checked independently. Question #2: I am not entirely clear on your question, if the reduction is negative, that means that there is an increase in runoff. If there is an increase in runoff, you will have to develop strategies to reduce the volume and/or flow rate to meet the LEED criteria for the credit. Again, the reduction may be different, depending on which credit you are evaluating. If you want to clarify the question, I may be able to provide a better response.