If you are using bitmap or RLE compression, some fragmentation occurs, the amount of which is based on individual database and operating system configurations and cannot be precisely predicted.
As a rough estimate, calculate 20% of the compressed database size (value DE from Table 247, Worksheet: Estimate of Disk Requirements for a Database) and write the result to the cell labeled DG in the same table.
Calculating fragmentation allowance assuming a compressed database size of 5,769,000,000 bytes:
5,769,000,000 bytes * .2 = 1,153,800,000 bytes