drainage area for sediment basin

If you are trying to figure out the required storage volume for a sediment basing, this has nothing to do with LEED boundaries or building footprints, it needs to hold the runoff that drains to it, and slowly release it (usually over the course of 7+ days). I like to use 3600 cf/ac as a starting point.

You're not calculating runoff. You are providing volume. 3600 cf per acre that drains to the sed basin

The 3600 cf/ac is an orientation Michael has provided for you to calculate how much volume would be needed to use on your site. The area you are asking about should be the size of the LEED Project Boundary, as this is the area that you need to drain during construction. You are essentially providing a buffer or a series of buffers to reduce sediment loss from your site. In terms of the rainfall you should use, the 2year/24hr design storm would be sufficient, but LEEDv2009 does not require a specific rainfall quantity to be used, as your contractor and civil engineer should already have done this based on local norms.