Logarithmic volume scaling
What is the problem you are facing?
From what I can see, you're using just a normal linear scale for the volume. The difference in perceived volume above 50% is barely distinguishable now.
Human ear perceives volume logarithmically, so I guess, the best option is to go with it. That way lower volumes can be actually more silent and the difference between 100% and 90% would be distinguishable now.