The application-specific volume solution makes sense.
However, my multimedia keyboard (not music keyboard by typing keyboard) has a volume knob, which is tied to the windows volume control. This makes it really easy to adjust volume if songs have different volumes, or if someone walks in the room. I love that volume control, since it's inches away from where my hand is.
I suppose if I moved my Scarlett closer to my multimedia keyboard I could just use its knob. However, at the moment it's near my studio equipment, which is about 4 feet away.