There's nothing that really says what the sensitivity scales against. 1.0, maybe? It'd be easy to just adjust the range so whatever number you want to try is the min or max sensitivity and peg your slider to it (that's what I did for FOV), but I just fiddled with the slider in the 0.05 to 0.5 range until I was comfortable. The original range Irrational uses is 0.1 to 4.0, which is a big range that gets unusably twitchy by the time I get to a third of the slider's bar.
Fortunately, the game does give you a check box to turn off mouse acceleration right off the bat.