Organizing include files
I've converted already some header files from C to use with nasm. To have a somewhat easy way to use these I've organized them into directories (which are almost the same as the header files for C).
To use them without troubles you should set the NASMENV to /[location of root directory]/includes/ (don't forget the trailing slash). You can use a shortcut to the location of your include files if you are intend to modify them often, as I do.
In that case install (as an example) the include files in your home directory. Make a directory /nasm somewhere else, create a shortcut of the include files directory in your home directory, copy to /nasm/ and rename it includes.
Your directory structure, as an example, can be organized like the picture below where the /nasm/includes points to /root/home/home-user/linux-nasm/Include-Files.
The advantage is when you keep your files on a subversion system like me, all your files are at the same place, you don't have to modify copies and your environment parameter string isn't too long to remember. It's up to you however how you organize your files.