The ideal process for us involves converting the installation .dmg from read-only to read/write and then modifying the Assisted Install.txt file as needed from there.
You generally want to have the installation files in a disk image. This keeps all the required files together in one place, but it also lets you keep the installer on a shared folder and run it over the network; .pkg files can't be run directly from a shared folder, but they can be run from a mounted disk image, even if the disk image is in a shared folder.
But the .dmg file provided by FileMaker is not writeable, so you can't edit the Assisted Install.txt file in-place. So we use Disk Utility on a Mac to convert the provided .dmg file from a read-only image to a "read/write disk image", and then we can edit the Assisted Install.txt right where it is.
Going this route saves you the time required to copy the installer first to the desktop, particularly if you're installing FileMaker Pro on multiple machines. Everyone can just mount the disk image, run the installer, and unmount the disk image when the installation is finished.
I hope this helps!