What you are doing in effect is "reserving" a place for that new data so that it will be written along side the existing data in a contiguous state and not written somewhere else on the disk that will require split I/O every time it needs to be accessed, and again when it is time to back that data up.
The right tool for the job in this case is Defrag.NSF, using the option to "Monitor Free Space". When the administrator selects this option and configures the free space desired for that particular database, Defrag.NSF will monitor that setting for the database and make sure that amount of space is always available for new data to be written. This happens during the automatic defrag maintenance schedule and if this setting is sufficient your nsf files will never fragment again.
The beauty of Defrag.NSF is that this setting will be continuously monitored, and any fragmentation that does occur due to data "overflow" will be promptly and automatically dealt with at the next scheduled Defrag.NSF maintenance run, along with the fresh allocation of the free space.
For more information check out our website at www.preemptive.com.au
Here is an example of a Database settings document