Confused by Reflex's mouse acceleration options? Completely understandable - there's a lot going on with all of the variables. Even worse - acceleration settings deeply depend on your mouse's DPI, so you can't just copy/paste someone else's settings and hope they work for you!
So, I wrote this little tool to give you a starting point. First, enter your mouse DPI and reflex game sensitivity (aka m_speed in console). Then go to into Reflex's "Options" menu, then "Mouse Settings" and copy the outputs this tool spat out (you'll need to check the box for "Enable Advanced Options").
Now that you've copied those settings into the menu, before you go in game do some quick wrist flicks and see if the little white circle can make it to the flat part of the curve at the top. Hopefully it can (and hopefully it doesn't reach it with very slow mouse movements). If not, tweak the acceleration value up or down until you can make it hit the flat part.
Next, go in game and see how it feels. It might feel like it's terribly high sensitivity. If so, change both of the post-scale settings to something like 0.6. If it's still too high, change it to 0.5 or something lower. Basically, tweak those values according to your preference and try to leave the other values alone. The "x" value is for horizontal movement, and the "y" value is for vertical. I have come to appreciate higher vertical sensitivity (I have 0.31 X and 0.5 Y) for easier rocket jumps.
If you end up liking the in game mouse acceleration options, keep in mind that povohat wrote an awesome driver that lets you have the same feeling in Windows and any other game. And this tool will let you copy your Reflex settings directly to the driver for the same acceleration curve and feeling.