Hey Andrew -
You should base the shape on the output of a calculated field, not your actual measure. Then you can take full control over what you want to see.
If [Measure] > 1000 Then "Good"
elseif [Measure] >500 AND [Measure} < 1000 "OK"
In the expression above, we now to expect one of three values. We can drag this field to our Shapes shelf, and code each string (Good, OK, Stinky) to a specific shape.
Hope this helps!