Ok I don't know if there's a lower one than 100%, but I'll just explain the script anyway. First you will need to hit windows key + R and type regedit, and go to the location pictured. You are trying to locate your display number Mine is
, that is my display. You will need to replace both locations that have that with your display name in the script.
See my recommended display is 150%, so for Time Crisis 5 I wanted to make it 100% for the game only. So I have mine set to -2 as you see below. Since yours recommended is 100% you will want a negative number. You can try -1 or -2. Notice there's another SetDpi under escape, this is where you want to it to go back to the original Dpi so it should be 0 for you. You also need to change your monitors display resolution where mine says 1920 x 1080. Also of course you will want to change the .exes under escape to your game's .exes names.
#NoEnv ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn ; Enable warnings to assist with detecting common errors.
SendMode Input ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir% ; Ensures a consistent starting directory.
run your game.exe here
Process,Close,Put correct game name here.exe
Run,taskkill /im "Put correct game name here.exe" /F
RegWrite, REG_DWORD, HKEY_CURRENT_USER\Control Panel\Desktop\PerMonitorSettings\VIZ10390_01_07E4_30^30BB376176AE7BF6A3A1FE1F935346BE, DpiValue, % dpi
RegWrite, REG_DWORD, HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\GraphicsDrivers\ScaleFactors\VIZ10390_01_07E4_30^30BB376176AE7BF6A3A1FE1F935346BE, DpiValue, % dpi
ChangeResolution(Screen_Width := 1920, Screen_Height := 1080, Color_Depth := 32) ; change these values for your screen resolution
DllCall( "EnumDisplaySettingsA", UInt,0, UInt,-1, UInt,&Device_Mode )
Return DllCall( "ChangeDisplaySettingsA", UInt,&Device_Mode, UInt,0 )