Multiple levels of spatial index gridding have a performance cost. They should only be used
when needed (e.g., when there is effectively two sets of data with different intrinsic sizes
within the table).
If you group the features to an overlay fishnet, then there is no reason to have more than
one grid size. The index grid size should be at least twice the size of the fishnet distance
(if it's a large net [e.g., 4x4 - 5x5 over the area], then you can go as small as 133% of the
fishnet, but never down to the size itself, since that would generate 4-9 index rows with
each feature -- far too many false positives in the index).
- V