Pirate Audio 3W minimum volume setting via Iris

Hello, I absolutely love the Pirate Audio 3W board and have it set up in stereo to run through a 1934 radio speaker and a 1923 horn speaker - sounds and looks great!
The problem I have is the minimum volume when set on the Iris web slider is a little too loud.
However if I adjust the start up mixer volume in mopidy.conf to 1%, it’s a good start volume.

[audio]
mixer_volume = 1

But if I use Iris scroll bars to increase the sound I can’t get back to this without a restart.
Is there a way of using the Iris ‘Add Command’ feature to somehow access the software mixer set_volume method?
I’ve searched for examples of how to use this interface, but struggle to find any written in the kind of numpty example I need :)
It seems the software mixer volume at 1% is a good lowest volume for my setup. What I really want is to change the volume control scale to be much more granular at the lower end. But I figure adding a button via the built in Iris functionality to set the volume level back to the start up 1% level would be a good workaround. Any idea if this can be done please and what the syntax would be ?
Thank you,
Matthew