I have discovered that (for those who really want to change this) the pscn value in the file private/var/db/.com.apple.iokit.graphics is related to the underscan value. When there is no underscan, pscn is 10000, and when underscan is full pscn goes to 8800 (if I remember right). The problem is that this file cannot be changed during normal operation of the computer and it appears to be overwritten whenever one boots in safe mode.
When I first got my TV monitor (2014 or so), I used the underscan function to show the entire screen. I recently discovered that I can remove the overscan on the TV side of things. Since then I have been desperately trying to find a way to get rid of the underscan value I added so long ago. I found that Users/[username]/Library/Preferences/ByHost/com.apple.windowserver.[random hex values].plist contains a lot of data about the display, this data can be changed, and all changes are accepted if you just reboot the computer. The problem is that it's really just the backend of System Preferences>Displays and it contains nothing about underscan. I finally got so fed up that I borrowed another person's Mac (happily she hadn't updated the OS recently), plugged it into my TV, changed the underscan value in System preferences, and used "find . -cmin -1" to find all files that had been changed in the last minute. Searching through this list brought me to the hidden file private/var/db/.com.apple.iokit.graphics, and repeatedly opening this file while changing the underscan value showed that pscn is the value that changes when the underscan changes. SUCCESS... kinda. private/var/db/.com.apple.iokit.graphics can't be changed, even by :wq! in vim.
In order to change private/var/db/.com.apple.iokit.graphics, I rebooted my computer and started up in safe mode, only to find that private/var/db/.com.apple.iokit.graphics had LOST ALL VALUES FOR MY TV, including the pscn of 9685. The only values that were still there were for my built-in monitor. When I then plugged in the TV, the underscan was gone and private/var/db/.com.apple.iokit.graphics showed a normal pscn of 10000 again. I restarted once more and the TV was properly scaled with pscn=10000.
All of this to say: If you really need to underscan, it may not be possible. If it is possible, you'll have to boot in safe mode, plug in your monitor, change the right pscn value in private/var/db/.com.apple.iokit.graphics, reboot in normal mode, and pray that the change sticks.
