A good sensitivity is usually around 30-60cm per 360. It depends on your grip (palm, fingertip, etc) and your aiming style (arm or wrist). There are plenty of professional/very high tier players across all types of games who use a very high/very low sens. What you mentioned is in the mid/low range.
Still haven't had an expert come through with concrete facts though : (. set it to 2.0 since l4d2 uses miles audio codec for deciding game sounds. if you want to try and use creatives hardware engine for processing sound via OpenAL then install Creative Alchemy add l4d2 to it (or any source game), set sound to 2.0 in game and add snd_legacy
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