I would be really surprised if this parameter was linear with power (or linear with voltage). I suggest you decrement in smaller steps and explore the response that way.
I also wouldn't be surprised if the spurious output of the transmitter changed with this setting. You might want to ask KI6ZUM. He probably knows.
Keep in mind that when you configure MMDVMHost, like in Pi-Star, takes a floating point number in the range of 0.0 to 100.0 then it scale that number to an 8-bit (1-byte) number (0-255) before setting it into the device, while with QnetModem, the range is 0-255. In MMDVMHost, the default value is 50, so I set the default value in QnetModem to 128.
Tom Early