Hiddedn blob pages

Hi: I've got a question about "blob-page-overhead". We create a blobspace with 27,815 blob pages. The blob page is sized to hold a 128x128 image. When asked to provide a utility for users to display the "number of images that can be inserted into the database", I simply counted the number of images already in the database and subtracted from the total number of blob pages. But the users are upset because the utility is saying that more images will fit in the database than can actually be inserted. I'm now guessing that there is some per-blob overhead being hidden that I need to adjust for in my calculations. Is this per-blob overhead a known ammount? The only other point that is bothersome is that the same application on identical equipment will have a different maximum image count. Any ideas?? Frank