Here's the explanation for what the iPresentInterval setting does:Btw found this at the Bethsoft official pc forums
http://forums.bethsoft.com/topic/1543742-fallout-4-pc-support-please-read/
I am running 144Hz with Gsync on. Gonna try setting it to 120Hz and iPresentInterval = 2.
http://help.bethesda.net/app/answers/detail/a_id/31653
In general, you are advised to keep the game's default settings and continue to play the game with a 60fps cap. The fps cap that the game uses is determined by the your monitor refresh rate (typically 60Hz or 120Hz), and dividing it by the "iPresentInterval" value.
Please note that if your monitor is set to 60Hz, the iPresentInterval value in your .INI file should be "1", and if your monitor is set to 120Hz, the iPresentInterval value should be "2". Setting it to "0" uncaps the framerate and can cause the issues mentioned above. If your monitor is set to a value larger than 120Hz (such as 144Hz), they are advised to set their desktop refresh rate to 120Hz instead, when playing the game.
So people saying that iPresentInterval = 2 sets the framerate vsync to 30 FPS are only correct if the monitor is locked to 60 Hz.