This is what I did:
Because the Axis only goes from 0 - 100, with no negative component, the sim can’t do this natively. So when the prop lever is fully aft, it is at 100. Spad detects when this is at 100, waits 100ms (which was the “magic” bit to make this work, then sets the “PROP_PITCH1_SET” to 4294963200. This is the number you see that variable get set to when fully feathered with the mouse.