Right I figured this one out (edit - probably not

). There are probably a couple of ways to fix it so it works, but I chose this one.
You need to set up the throttle axis as normal by pushing up or down on the throttle while the control binder (in game) waits on an input.
Then go into your bindings folder (C:\Users\
YourName\AppData\Local\Frontier Developments\Elite Dangerous\Options\Bindings) and open your .binds file with notepad (I have saved mine as Custom HOTAS).
After that you need to change the "ThrustmasterTFlightHOTASX" device to "FEEDFACE" DeviceIndex="0"
This is the information you need to change -
<ThrottleAxis>
<Binding Device="FEEDFACE" DeviceIndex="0" Key="Joy_ZAxis" />
<Inverted Value="1" />
<Deadzone Value="0.00000000" />
</ThrottleAxis>
Edit - It appears to work around the 50% point however (in both directions). Is that what you wanted or did you want it slower at 0% and catching up at 100%?