If possible, could you share a Sample Workbook, or perhaps a Screenshot of your Viz?
I have an idea of how I could do this, but it will depend on how things are structured.
I have done something similar. I used an if statement to cap the maximum and minimum value that was on the color shelf while having the original value show on details shelf and tooltip.
It is a simple if statement below if basically what I used
IF Var1<= 300 then 300
else if Var1>=5000 then 5000
Modified you could do something like this calculated field.
IF Var1>=(max(Var1) - Min(var1))*.5 then(max(Var1) - Min(var1))*.5
Hope this helps.