You must move your slider down, and then up. It becomes reasonably intuitive with a small amount of practice :)

Could have feedback with a LED ramp next to slider, showing the discrepancy...

Down then up wouldn't be too bad, but up then down could very much be, in certain situations.

I would put a safety routine to let the software update only when the hardware is within a certain amount from the actual value, also giving visual feedback of the difference, so that the user could align hardware position to software position without any sudden changes.

You put the slider all the way down to 0, which is then mapped the the range from 0-0 and then you move it back up?!

Once the fader matches software, software follows the fader.