By painted layer you mean the Black and White image that is plugged into Factor of the Mix Shader node?
The opacity is controlled by grayscale values of that image going from 0 to 1, black to white. It either shows you one layer, or the other, or mix of the two. So black (factor = 0) means transparent, white (factor = 1) means opaque, and everything grey is a mix.
So getting it more transparent is a matter of changing white value in that image to something grey. There are tons nondestructive ways to do it. [A destructive option is to just choose a grey color and paint with it]
Here’s one: with Map Range node you can lower the maximum value of the image.