You are correct in your assertion. Try this site:
In this solution when creating a sparsebundle you'll need to specify a file size so when entering in the information in your console you'll need to enter this:
hdiutil create -fs HFS+J -size [Size_in_GB]g -volname "Backup of [Your_Computer_Name]" [Your_Computer_Name]_[Your_ethernet_MAC_Address].sparsebundle
The following is the legend for variables in that command line DO NOT ENTER IT IN CONSOLE:
[Size_in_GB] = Limit of sparsebundle created in GB
[Your_Computer_Name] = Found by clicking on the Gray Apple (dependent on your OS version) and going to "About This Mac"->More Info... (The computer name should be in the title bar)
[Your_ethernet_MAC_Address] = Found by going into System Preferences->Network->Ethernet controller->Advanced...->Ethernet (Make sure you use your LAN not your WLAN address)
The sparsebundle will be created on your Desktop or User folder at which point you should copy it to the area on your NAS you wish to use for Time Machine.