Perceived loudness can be calculated as
k^(1/3.5) where k is a ratio of power (or squares of voltage)
in your case (0.5^2/0.3^2)^(1/3.5) =1.34
0.5mV will be 34% louder than 0.3mV
Another typical example 200W amp will be louder than 100W amp by (200W/100W)^(1/3.5)=1.219 times (21.9%)
k^(1/3.5) where k is a ratio of power (or squares of voltage)
in your case (0.5^2/0.3^2)^(1/3.5) =1.34
0.5mV will be 34% louder than 0.3mV
Another typical example 200W amp will be louder than 100W amp by (200W/100W)^(1/3.5)=1.219 times (21.9%)