Hi! Unfortunately there's no way to make that happen with midi mixer natively.
You could use Voicemeeter (you only need the basic free one). To limit the output volume to your device, you'll have to do a couple things. ALSO VM might introduce some latency depending on system and configuration settings.
First, switch your Windows default output device to Voicemeeter VAIO (This will need to be redone every time an audio device is plugged/unplugged to your PC). Then set the A1 device to your main device (choose your ASIO device if you have one. ASIO4ALL or similar softwares will not work correctly) and slide the output fader down to what you want the maximum to be. Finally, click the A button of the VAIO device in VM to route it to the output device.
Any adjustments you make to volume on Windows (and midi mixer in turn) will be finer adjustments and 100% volume will be softer, but only for the device you selected.
NOTE: I believe applications that are set to go directly to your A1 device (not through VM) will not be limited by voicemeeter in non-ASIO configurations, but I haven't tested it.