You already can. Just click into the number box you want, in the property you're wanting to set, and type the number you want. This works on all clip properties, like if you're angling the picture, for example, or adjusting the volume, like in your example.
I have the same issue - I need a % of 3-4 but when I set the property box to this level - it does not change on the timeline - or am I mistaken in that you cannot set the level like this. Every time I try to set the level with the mouse - the level jumps - is there a way to stabilize this setting?