Nice to hear that is exact what you looking for, but in my case I would not prefer the percentage from the current level. I would prefer more for a example like: If your current volume is 23, in that case it will set the volume
for each click on the button 10+/- like 33,43,53,... or 13,3,0 this makes more sense for me
If you search more for this, let me know
or maybe you can figured it out by your self with my given command.