The file is located

"%LOCALAPPDATA%\Warframe\EE.cfg"

You can find this file by searching "%localappdata%" in windows start menu.

From here find the "warframe" folder > open it > than open "EE.cfg" (notepad will work)

Create a copy! incase you fudge it up!

Now either hit ctrl + f and look for "look"

You will notice you can only find the section for the x axis (we will fix this)
Here change the value for "scale" to what you would like this can only go down to 0.01 in theory.

Than copy from ...

LOOK_X={
{
key=EN_MOUSE_X
inputMode=IM_NORMAL
invert=0
scale=0.010043
inputFilter=""
strictFiltering=0
slot=0
}

Than you must paste it outside of the last div, below

key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/SpaceBattle/SpaceTennoInputFilter
strictFiltering=0
slot=0
}
}
[paste here within the document]
Than dont forget to close it with "}"

Below is a correct example.

LOOK_X={
{
key=EN_MOUSE_X
inputMode=IM_NORMAL
invert=0
scale=0.010043
inputFilter=""
strictFiltering=0
slot=0
}
,
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/TennoInputFilter
strictFiltering=0
slot=0
},
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/TennoEquippedMeleeFilter
strictFiltering=0
slot=0
},
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/SpaceBattle/SpaceTennoInputFilter
strictFiltering=0
slot=0
}
}
LOOK_Y={
{
key=EN_MOUSE_Y
inputMode=IM_NORMAL
invert=0
scale=0.010043
inputFilter=""
strictFiltering=0
slot=0
}
}


____________________________________
This is an incorrect example
And WILL lead to all binds being reset...
you might be able to remedy by undoing in the notepad file (ctrl z) back to original state. Or by deleting the file and rename the 'back up' to EE.cfg

LOOK_X={
{
key=EN_MOUSE_X
inputMode=IM_NORMAL
invert=0
scale=0.010043
inputFilter=""
strictFiltering=0
slot=0
}
LOOK_Y={
{
key=EN_MOUSE_Y
inputMode=IM_NORMAL
invert=0
scale=0.010043
inputFilter=""
strictFiltering=0
slot=0
}
,
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/TennoInputFilter
strictFiltering=0
slot=0
},
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/TennoEquippedMeleeFilter
strictFiltering=0
slot=0
},
{
key=EN_GAMEPAD_RX
inputMode=IM_NORMAL
invert=0
scale=1
inputFilter=/Lotus/Types/Player/SpaceBattle/SpaceTennoInputFilter
strictFiltering=0
slot=0
}
}

The in game sens is decoupled to this but works as a multiplier so if 0.01 isnt low enough lower your in game sens to something lower.

Or you can utilise
www.mouse-sensitivity.com/
This will let you calculate the sens to any other game