My experience, and a brief experiment, suggests that the Z Scale control works in the opposite direction that the documentation suggests. Fusion assumes that a typical Z buffer will contain values from 0 - 1000, if I recall correctly. If that is true, then leaving Z Scale at 200 causes the tool to operate comfortably. If your range is much smaller than that, say 1 - 10 like my test image, reducing the Z Scale seems to make it work better.
My experiment image has a buffer with actual values from 3 - 8. I attempted to get the same blur quality from that and from a version where I'd gained the Z by 200, resulting in values from 600 - 1600. Here's a screenshot of my results:
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The Color Inspector values in each Viewer are drawn from similar locations. As you can see, when the Z values are low, my Z Scale control is set at 0.2. When they are high, it is set at 91.
The Z Scale isn't really something you need to adjust often, though. Since it's a "set it and forget it" control, I don't think there's much problem with having to manually enter numbers there. The Focal Point slider, though, operates on the pre-scaled values, and since its range is -1000 - 1000, a scene with small Z values causes that slider to be too sensitive.
There is, fortunately, a way to reduce that sensitivity. If you right-click a tool and choose "Edit Controls…" you will get an interface where you can adjust the behavior of many controls.
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You just need to choose the control you wish to edit and change the Range values. Also switch the Page to Controls so it doesn't wind up on a new User tab. You could do the same with the ZScale slider if you wish.